<PAGE>

                                                                     EXHIBIT 4.8

NEITHER THE SECURITY EVIDENCED BY THIS WARRANT NOR THE SECURITIES ISSUABLE UPON
EXERCISE OF THIS WARRANT HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
AS AMENDED. SUCH SECURITIES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF
SUCH REGISTRATION UNLESS THE COMPANY RECEIVES AN OPINION OF LEGAL COUNSEL FOR
THE HOLDER OF SAID SECURITIES THAT IS REASONABLY ACCEPTABLE TO THE COMPANY,
STATING THAT SUCH SALE OR TRANSFER IS EXEMPT FROM THE REGISTRATION REQUIREMENTS
OF THE SAID ACT OR THE COMPANY OTHERWISE SATISFIES ITSELF THAT SUCH TRANSACTION
IS EXEMPT FROM REGISTRATION.

                             COMMON STOCK WARRANT
                                      OF
                              CAIS INTERNET, INC.

          THIS CERTIFIES THAT, subject to the terms and conditions of this
Warrant, for the consideration of $1.00, the receipt of which is hereby
acknowledged, Microsoft Corporation, a Washington corporation, or its successors
and assigns (the "Holder"), is entitled to purchase, at any time and from time
to time on or after the date hereof, shares of Common Stock, par value $.01 per
share (the "Common Stock"), of CAIS Internet, Inc., a Delaware corporation (the
"Company"), from the Company in such number and at such price as determined in
accordance with this Warrant.

          Upon delivery of this Warrant (with the Notice of Exercise in the form
attached hereto as Exhibit A), together with payment of the Warrant Price (as
defined below) for the shares of Common Stock to be issued, which payment may be
made by converting this Warrant, or any portion thereof, pursuant to Section 3
below ("Warrant Conversion"), at the principal office of the Company or at such
other office or agency as the Company may designate by notice in writing to the
Holder hereof, the Holder shall be entitled to receive a certificate or
certificates for the shares of Common Stock so purchased.  All shares of Common
Stock that may be issued upon the exercise of this Warrant will, upon issuance,
be fully paid and nonassessable and free from all taxes, liens and charges with
respect thereto.  References herein to Hotel Portal, Hotel Rooms and Qualified
Units shall have those definitions as set forth in a certain Internet Access and
Portal Development and Co-Marketing Agreement between the Company and the Holder
(the "Internet and Portal Agreement").

          This Warrant is subject to the following terms and conditions:

          1.   Term of Warrant. Subject to the vesting and termination
provisions set forth in Section 2 hereof, this Warrant may be exercised in whole
or in part, at any time and from time to time on or after the date hereof;
provided, however, that this Warrant shall expire to the extent then unexercised
--------  -------
as of 5:00 p.m., eastern time on April 28, 2005.
<PAGE>

     2.   Number of Warrants; Vesting.
          ---------------------------

          (a)  Number of Warrants. Subject to the terms and conditions set forth
herein, this Warrant shall entitle the Holder to purchase up to 900,000 shares
of Common Stock at the warrant price(s) ("Warrant Prices(s) set forth in Section
2(b). The number of shares issuable upon exercise of the Warrant and the Warrant
Price(s) are subject to adjustment pursuant to Section 4 hereof.

          (b)  Vesting. This Warrant will vest, in whole or in part (but not for
a fraction of a share), upon the occurrence of the events described below in the
amounts and strike price set forth below:

     (i)  The Warrant shall vest with respect to up to 300,000 shares of Common
Stock, either a Warrant Price of $45 per share, if CAIS has not installed high
speed connectivity employing the Hotel Portal as the default start page in at
least 725,000 Qualified Units by December 31, 2001.  The number of shares vested
is based on 300,000 less a number determined by taking the ratio of actual
Qualified Units as of December 31, 2001 to 725,000 and multiplying that number
by 300,000 (with warrants to purchase fractional shares of Common Stock rounded
to the nearest whole share).  For example, if 500,000 Qualified Units are
installed by December 31, 2001, then 93,103 warrants at $45 per share will so
vest.  If so vested, such portion of this Warrant may be exercised beginning on
a day five days after the receipt of the CAIS Certificate or, in the event of a
Dispute, five days after resolution of the Dispute.  If such milestone has been
met, such portion of this Warrant shall terminate.

               (ii)   The Warrant shall vest with respect to up to 300,000
shares of Common Stock, with a Warrant Price of $55 per share, if CAIS has not
installed high speed connectivity employing the Hotel Portal as the default
start page in at least 925,000 Qualified Units by December 31, 2002. The number
of shares vested is based on 300,000 less a number determined by taking the
ratio of actual Qualified Units as of December 31, 2002 to 925,000 and
multiplying that number by 300,000 (with warrants to purchase fractional shares
of Common Stock rounded to the nearest whole share). If so vested, such portion
of this Warrant may be exercised beginning on a day five days after the receipt
of the CAIS Certificate or, in the event of a Dispute, five days after
resolution of the Dispute. If such milestone has been met, such portion of this
Warrant shall terminate

               (iii)  The Warrant shall vest with respect to up to 300,000
shares of Common Stock, with a Warrant Price of $65 per share, if CAIS has not
installed high speed connectivity employing the Hotel Portal as the default
start page in at least 1,200,000 Qualified Units by December 31, 2003. The
number of shares vested is based on 300,000 less a number determined by taking
the ratio of actual Qualified Units as of December 31, 2003 to 1,200,000 and
multiplying that number by 300,000 (with warrants to purchase fractional shares
of Common Stock rounded to the nearest whole share). If so vested, such portion
of this Warrant may be exercised beginning on a day five days after the receipt
of the CAIS Certificate or, in the event of a Dispute, five days after
resolution of the Dispute. If such milestone has been met, such portion of this
Warrant shall terminate.

                                       2
<PAGE>

               (iv)   On or before January 10, 2002, 2003 and 2004 respectively,
CAIS shall deliver a certificate to the Holder setting forth the number of such
installations that existed on December 31, of the prior year (the "CAIS
Certificate"). Upon receipt of the CAIS Certificate, the Holder may determine to
independently audit such calculations. In the event Holder decides to audit
CAIS' calculations, it will deliver to CAIS within fifteen days of the receipt
of the CAIS Certificate an "Audit Notice." Any audit will be conducted by an
independent party and at Holder's expense. If, upon the conclusion of the audit
the parties disagree on the calculations (a "Dispute"), they shall at first
attempt through good faith negotiations to resolve the Dispute. Any Dispute not
so resolved shall be submitted to final and binding arbitration in King County,
Seattle, Washington, which arbitration shall, except as herein specifically
stated, be conducted in accordance with the CPR Non-Administered Arbitration
Rules then in effect.

     3.   Cashless Exercise of Warrant.  Notwithstanding any provisions herein
to the contrary, if the Net Issue Fair Market Value (as defined below) of one
share of Common Stock is greater than the Warrant Price for one share of Common
Stock (at the date of calculation, as set forth below), in lieu of exercising
this Warrant for cash, the Holder may elect to receive shares of Common Stock
equal to the value (as determined below) of this Warrant (or the portion thereof
being canceled) by surrender of this Warrant at the principal office of the
Company, in which event the Company shall issue to the Holder that number of
shares of Common Stock computed using the following formula:

                              CS = WCS (NIFMV-WP)
                                   --------------
                                     NIFMV

     Where:

      CS       equals the number of shares of Common Stock to be issued to the
               Holder;

      WCS      equals the number of shares of Common Stock purchasable under the
               Warrant or, if only a portion of the Warrant is being exercised,
               the portion of the Warrant being exercised (at the date of such
               calculation);

      NIFMV    equals the Net Issue Fair Market Value (as defined below) of one
               share of Common Stock (at the date of such calculation); and

      WP       equals the per share Warrant Price (as adjusted to the date of
               such calculation pursuant to Section 4) of the Warrant.

As used in this Section 3, the term "Net Issue Fair Market Value" of each Share
as of any date shall be the average of the closing sale prices posted in respect
of the Common Stock in the NASDAQ Stock Market's automated dealer quotation
system (or, as reported on any exchange on which the Common Stock is then
trading) for the twenty (20) consecutive trading days ending on the date prior
to such exercise.

                                       3
<PAGE>

     4.   Adjustment of Number of Shares and Warrant Price.  The number and kind
of Shares purchasable upon the exercise of the Warrant and the Warrant Price
shall be subject to adjustment from time to time in accordance with the
following provisions:

          (a)  Special Definitions.  For purposes of this Section 4, the
following definitions shall apply:

               (i)   "Option" shall mean rights, options or warrants to
     subscribe for, purchase or otherwise acquire Common Stock or Convertible
     Securities, excluding rights or options granted to employees, vendors,
     officers, directors and executives of, and consultants or shareholders to,
     the Company in an amount not exceeding the number of Reserved Employee
     Shares.

               (ii)  "Convertible Securities" shall mean any evidences of
     indebtedness, shares or other securities directly or indirectly convertible
     into or exchangeable for Common Stock.

               (iii) "Additional Shares of Common Stock" shall mean all shares
     of Common Stock issued (or, pursuant to Section 4(c) below, deemed to be
     issued) by the Company at any time while this Warrant remains outstanding
     and unexpired, other than Reserved Employee Shares and other than shares of
     Common Stock issued or issuable:

                    (i)   upon the exercise of Options;

                    (ii)  by reason of the issuance of Series D Preferred Stock
               (as defined below) or Series E Preferred Stock (as defined below)
               pursuant to the Preferred Stock Purchase Agreement between the
               Company and CII Ventures LLC, dated as of December 20, 1999;

                    (iii) by reason of the issuance of Series F Convertible
               Participating Preferred Stock, par value $.01 per share ("Series
               F Preferred Stock") pursuant to the Series F Preferred Stock
               Purchase Agreement between the Company and Microsoft Corporation
               dated as of April __, 2000;

                    (iv)  by reason of the issuance of Series G Convertible
               Preferred Stock, par value $.01 per share ("Series G Preferred
               Stock"), pursuant to the Series G Preferred Stock Purchase
               Agreement between the Company and 3 Com Corporation, dated as of
               March 20, 2000;

                    (v)   as a dividend or distribution on Series F Preferred
               Stock, Series G Preferred Stock, Series C Preferred Stock, par
               value $.01 per share ("Series C Preferred Stock"), Series D
               Convertible Participating Preferred Stock, par value $.01 per
               share ("Series D Preferred Stock"), Series E Convertible
               Participating Preferred Stock, par value $.01 per share ("Series
               E Preferred Stock"), or upon conversion of shares of Series

                                       4
<PAGE>

               C Preferred Stock, Series D Preferred Stock, Series E Preferred
               Stock, Series F Preferred Stock or Series G Preferred Stock;

                    (iv)   pursuant to warrants issued by the Company pursuant
               to (a) the Common Stock Warrant dated as of October 27, 1999 and
               the Series C Preferred Stock Purchase Agreement, dated as of
               September 29, 1999 between the Company and U.S. Telesource, Inc.
               (the "Qwest Warrant"), (b) the Warrant Agreement dated as of
               September 4, 1998 among the Company, Cleartel Communications,
               Inc., CAIS, Inc. and ING (U.S.) Capital Corporation, Inc. (the
               "ING Warrant Agreement"), (c) the Series A Preferred Stock and
               Warrant Purchase Agreement dated as of February 19, 1999 among
               the Company and the several purchasers set forth therein, (d) the
               Warrant to Purchase Common Stock issued pursuant to the First
               Amendment to the Master License Agreement, dated as of April 23,
               1999, among the Company, CAIS, Inc., and Hilton Hotels
               Corporation (the "Hilton Warrant"), (e) the Master Agreement for
               Hotel Internet Service dated as of January 1, 2000, between CAIS,
               Inc. and Bass Hotels and Resorts, Inc. and (f) the Common Stock
               Warrant dated as of April __, 2000 for the purchase of 600,000
               shares of Common Stock (subject to adjustment), issued to
               Microsoft Corporation;

                    (v)    by reason of a dividend, stock split, split-up or
               other distribution on shares of Common Stock;

                    (vi)   to a corporation, partnership or other entity with
               which the Company is seeking to establish a partnership, joint
               venture or other business relationship when the total number of
               shares of Common Stock so issuable or issued does not exceed
               1,000,000 shares (as appropriately adjusted for any stock
               dividends, combinations, splits or the like with respect to
               shares of Common Stock), provided the Company receives at least
               95% of Fair Market Value for such shares;

                    (vii)  in connection with any high-yield debt financing
               undertaken by the Company, not to exceed 2,000,000 shares of
               Common Stock in the aggregate, provided that the Company receives
               at least 95% of Fair Market Value for such shares (or, in the
               case of Convertible Securities, the conversion or exercise price
               therefor is at least 95% of the Fair Market Value of the Common
               Stock on the date of issuance of the Convertible Securities;

                    (viii) pursuant to the Agreement and Plan of Merger among
               the Company, Business Anywhere USA, Inc., CIBA Merger Corp., Kim
               Kao, and Amy Hsiao dated September 7, 1999, including without
               limitation, the conversion of Business Anywhere options into
               options to acquire Common Stock and the issuance of shares of
               Common Stock upon the

                                       5
<PAGE>

               exercise thereof, not to exceed 288,371 shares of Common Stock in
               the aggregate;

                    (ix)   in connection with the acquisition by the Company of
               the securities or assets of another corporation, partnership or
               other entity, provided the Company receives at least 95% of Fair
               Market Value for such shares; and

                    (x)    pursuant to the Agreement and Plan of Merger among
               the Company, CIAM Corp. and Atcom, Inc. dated August 4, 1999, as
               amended, including without limitation, the conversion of Atcom
               options into options to acquire Common Stock as described
               therein, the issuance of shares of Common Stock upon the exercise
               thereof and the issuance of Common Stock constituting "Contingent
               Consideration" as defined therein, not to exceed 2,654,826 shares
               of Common Stock in the aggregate.

               (iv) "Reserved Employee Shares" shall mean shares of Common Stock
     issued to employees, officers, directors, shareholders and executives of,
     and consultants or vendors to, the Company of up to: (i) 5,000,000 shares
     (as appropriately adjusted for any stock dividends, combinations, splits or
     the like with respect to shares of Common Stock), plus such additional
     number of shares of Common Stock issued or deemed issued for like purposes
     as shall be approved by the Holder; plus (ii) shares reserved, as of the
     date hereof, for issuance upon the exercise of outstanding Options to
     purchase up to 3,031,495 shares of Common Stock presently held by certain
     management employees of the Company.  Such Reserved Employee Shares shall
     be issued, at any time, and from time to time, under such arrangements,
     contracts or plans as are recommended by the Company's management and
     approved by the Board.

               (v)  "Rights to Acquire Common Stock" (or "Rights") shall mean
     all rights issued by the Company to acquire Common Stock whether by
     exercise of a warrant, option or similar call, or conversion of any
     existing instruments, in either case for consideration fixed, in amount or
     by formula, as of the date of issuance.

               (vi) "Fair Market Value" of any property shall mean the fair
     market value thereof as determined in good faith by the Board of Directors
     of the Company (the "Board"); provided, however, that the value of any
                                   --------  -------
     securities will be determined as follows:

                    (i) Securities not subject to investment letter or other
               similar restrictions on free marketability covered by (ii) below:

                        (A) If traded on a securities exchange or through the
               Nasdaq National Market, the value shall be deemed to be the
               average of the closing prices of the securities on such quotation
               system over the thirty (30) day period ending three (3) days
               prior to the closing;

                                       6
<PAGE>

                        (B) If actively traded over-the-counter, the value shall
               be deemed to be the average of the closing bid or sale prices
               (whichever is applicable) over the thirty (30) day period ending
               three (3) days prior to the closing; and

                        (C) If there is no active public market, the value shall
               be the fair market value thereof, as mutually determined by the
               Board and the Holder.

                  (ii)  The method of valuation of securities subject to
               investment letter or other restrictions on free marketability
               (other than restrictions arising solely by virtue of a
               stockholder's status as an affiliate or former affiliate) shall
               be to make an appropriate discount from the market value
               determined as above in (i)(A), (B) or (C) to reflect the
               approximate fair market value thereof, as mutually determined by
               the Board and the Holder.

          (b)  No Adjustment of Warrant Price. The number of Shares purchasable
upon the exercise of the Warrant shall not be adjusted, by adjustment in the
Warrant Price thereof, unless the Fair Market Value of the consideration per
share (determined pursuant to Section 4(e) below) received by the Company for an
Additional Share of Common Stock issued or deemed to be issued by the Company is
less than the greater of 95% of the Fair Market Value per share of the Common
Stock or the applicable Warrant Price in effect on the date of, and immediately
prior to, the issue of such additional shares, or if prior to such issuance, the
Company receives written notice from the Holder, agreeing that no such
adjustment shall be made as the result of the issuance of Additional Shares of
Common Stock.

          (c)  Issue of Securities Deemed Issue of Additional Shares of Common
Stock. If the Company at any time or from time to time while this Warrant
remains outstanding and unexpired shall issue any Options or Convertible
Securities or Rights to Acquire Common Stock, then the maximum number of shares
of Common Stock (as set forth in the instrument relating thereto without regard
to any provision contained therein for a subsequent adjustment of such number)
issuable upon the exercise of such Options, Rights to Acquire Common Stock or,
in the case of Convertible Securities, the conversion or exchange of such
Convertible Securities, shall be deemed to be Additional Shares of Common Stock
issued as of the time of such issue; provided, however, that Additional Shares
of Common Stock shall not be deemed to have been issued unless the Fair Market
Value of the consideration per share (determined pursuant to Section 4(e)
hereof) received by the Company for such Additional Shares of Common Stock would
be less than the greater of 95% of the Fair Market Value per share of Common
Stock or the applicable Warrant Price in effect on the date of and immediately
prior to such issue, or such record date, as the case may be, and provided,
further, that in any such case:

               (i)  No further adjustment in the Warrant Price shall be
     made upon the subsequent issue of shares of Common Stock upon the exercise
     of such Options, Rights or conversion or exchange of such Convertible
     Securities;

                                       7
<PAGE>

               (ii)   Upon the expiration or termination of any unexercised
     Option, Right or Convertible Security, the Warrant Price shall be adjusted
     immediately to reflect the applicable Warrant Price which would have been
     in effect had such Option, Right or Convertible Security (to the extent
     outstanding immediately prior to such expiration or termination) never been
     issued; and

               (iii)  In the event of any change in the number of shares of
     Common Stock issuable upon the exercise, conversion or exchange of any
     Option, Right or Convertible Security, including, but not limited to, a
     change resulting from the anti-dilution provisions thereof, the Warrant
     Price then in effect shall forthwith be readjusted to such Warrant Price as
     would have been obtained had the Warrant Price adjustment that was
     originally made upon the issuance of such Option, Right or Convertible
     Security, which were not exercised or converted prior to such change, been
     made upon the basis of such change, but no further adjustment shall be made
     for the actual issuance of Common Stock upon the exercise or conversion of
     any such Option, Right or Convertible Security.

          (d)  Adjustment of Warrant Price upon Issuance of Additional Shares of
Common Stock.  If the Company shall at any time while this Warrant is
outstanding and unexpired issue Additional Shares of Common Stock (including
Additional Shares of Common Stock deemed to be issued pursuant to Section 4(c),
but excluding shares issued as a dividend or distribution as provided in Section
4(g) or upon a stock split or combination as provided in Section 4(f)), without
consideration, or for a consideration per share less than the greater of 95% of
the Fair Market Value per share of Common Stock or the applicable Warrant Price
in effect on the date of and immediately prior to such issue, or without the
requisite notice contemplated by Section 4(b) hereof, then and in such event,
such Warrant Price shall be reduced, concurrently with such issue, to a price
(calculated to the nearest cent) determined by multiplying such Warrant Price by
a fraction, the numerator of which shall be the number of shares of Common Stock
outstanding immediately prior to such issue plus the number of shares of Common
Stock which the aggregate consideration received by the Company for the total
number of Additional Shares of Common Stock so issued would purchase at the
greater of 95% of the Fair Market Value per share of Common Stock or such
Warrant Price; and the denominator of which shall be the number of shares of
Common Stock outstanding immediately prior to such issue plus the number of such
Additional Shares of Common Stock so issued.

     Notwithstanding the foregoing, the applicable Warrant Price shall not be
reduced if the amount of such reduction would be an amount less than $.03, but
any such amount shall be carried forward and reduction with respect thereto made
at the time of and together with any subsequent reduction which, together with
such amount and any other amount or amounts so carried forward, shall aggregate
$.03 or more.

          (e)  Determination of Consideration.  For purposes of this Section 4,
the Fair Market Value of the consideration received by the Company for the issue
of any Additional Shares of Common Stock shall be computed as follows:

               (i)  Cash and Property.  Such consideration shall:
                    -----------------

                                       8
<PAGE>

                     (i)   insofar as it consists of cash, be computed at the
               aggregate of cash received by the Company, excluding amounts paid
               or payable for accrued interest or accrued dividends;

                     (ii)  insofar as it consists of property other than cash,
               be computed at the Fair Market Value thereof at the time of such
               issue, as determined in good faith by the Board; and

                     (iii) in the event Additional Shares of Common Stock are
               issued together with other shares or securities or other assets
               of the Company for consideration which covers both, be the
               proportion of such consideration so received, computed as
               provided in clauses (i) and (ii) above, as determined in good
               faith by the Board.

               (ii)  Options, Rights and Convertible Securities.  The
                     ------------------------------------------
     consideration per share received by the Company for Additional Shares of
     Common Stock deemed to have been issued pursuant to Section 4(c), relating
     to Options, Rights and Convertible Securities, shall be determined by
     dividing

                     (i)   the total amount, if any, received or receivable by
               the Company as consideration for the issue of such Options,
               Rights or Convertible Securities, plus the minimum aggregate
               amount of additional consideration (as set forth in the
               instruments relating thereto, without regard to any provision
               contained therein for a subsequent adjustment of such
               consideration) payable to the Company upon the exercise of such
               Options, Rights or the conversion or exchange of such Convertible
               Securities, by

                     (ii)  the maximum number of shares of Common Stock (as set
               forth in the instruments relating thereto, without regard to any
               provision contained therein for a subsequent adjustment of such
               number) issuable upon the exercise of such Options, Rights or the
               conversion or exchange of such Convertible Securities.

          (f)  Adjustment for Stock Splits and Combinations.  If at any time or
from time to time while this Warrant remains outstanding and unexpired, the
Company shall effect a subdivision of the outstanding Common Stock, the Warrant
Price then in effect immediately before that subdivision shall be
proportionately decreased.  If at any time or from time to time while this
Warrant remains outstanding and unexpired, the Company shall combine the
outstanding shares of Common Stock, the Warrant Price then in effect immediately
before the combination shall be proportionately increased.  Any adjustment under
this paragraph shall become effective at the close of business on the date that
the subdivision or combination becomes effective.

          (g)  Adjustment for Certain Dividends and Distributions.  In the event
that, while this Warrant remains outstanding and unexpired, the Company at any
time or from time to

                                       9
<PAGE>

time shall make or issue a dividend or other distribution payable in Additional
Shares of Common Stock, then and in each such event the Warrant Price shall be
decreased as of the time of such issuance, by multiplying the Warrant Price by a
fraction, the numerator of which shall be the total number of shares of Common
Stock issued and outstanding immediately prior to the time of such issuance, and
the denominator of which shall be the total number of shares of Common Stock
issued and outstanding immediately prior to the time of such issuance plus the
number of shares of Common Stock issuable in payment of such dividend or
distribution.

          (h)  Adjustments for Other Dividends and Distributions.  In the event
that, while this Warrant remains outstanding and unexpired, the Company at any
time, or from time to time shall make or issue, a dividend or other distribution
payable in securities of the Company other than shares of Common Stock, then and
in each such event provision shall be made so that the Holder shall receive upon
the exercise of the Warrant in addition to the number of Shares receivable
thereupon, the amount of securities of the Company that the Holder would have
received had the Warrant been exercised with respect to the Shares on the date
of such event and had thereafter, during the period from the date of such event
to and including the date on which the Warrant was exercised, retained such
securities receivable by the Holder as aforesaid during such period given
application to all adjustments called for during such period.

          (i)  Reclassification, Consolidation or Merger. In case of any capital
reorganization, reclassification or change of outstanding securities of the
class issuable upon exercise of the Warrant (other than as a result of a
subdivision, split, combination or stock dividend), or in case of any
consolidation or merger of the Company with or into another entity, the Company,
or such successor entity, as the case may be, shall execute a new Warrant, with
substantially the same terms as this Warrant, or amend this Warrant, to provide
that the Holder shall have the right to exercise such new Warrant or amended
Warrant and procure upon such exercise in lieu of the Common Stock theretofore
issuable upon exercise of this Warrant the kind and amount of shares of stock,
other securities, money and/or property receivable upon such reorganization,
reclassification, change, consolidation or merger by the Holder as if this
Warrant had been fully exercised immediately prior to such event. Any such new
Warrant shall provide for adjustments which shall be as nearly equivalent as may
be practicable to the adjustments provided for in this Section 4. The provisions
of this subsection (a) shall similarly apply to successive reorganizations,
reclassifications, changes, consolidations and mergers.

          (j)  Certificate as to Adjustments.  Upon the occurrence of each
adjustment or readjustment of the Warrant Price pursuant to this Section 4, the
Company at its expense shall promptly compute such adjustment or readjustment in
accordance with the terms hereof and furnish to the Holder a certificate setting
forth such adjustment or readjustment and showing in detail the facts upon which
such adjustment or readjustment is based and shall file a copy of such
certificate with its corporate records.  The Company shall, upon the reasonable
written request of the Holder, furnish or cause to be furnished to the Holder a
similar certificate setting forth (i) such adjustments and readjustments, (ii)
the Warrant Price then in effect, and (iii) the number of Shares and the amount,
if any, of other property which then would be received upon the exercise of the
Warrant.

                                       10
<PAGE>

     5.   Notices.  Upon any adjustment of the Warrant Price and any increase or
decrease in the number of Shares purchasable upon the exercise of this Warrant,
then, and in each such case, the Company, within 30 days thereafter, shall give
written notice thereof to the registered holder of this Warrant (the "Notice").
The Notice shall be mailed to the address of such holder as shown on the books
of the Company; and shall state the Warrant Price as adjusted and the increased
or decreased number of shares purchasable upon the exercise of this Warrant,
setting forth in reasonable detail the method of calculation of each.

     6.   Transfer and Exchange of the Warrant and Shares.  When this Warrant or
Shares are presented to the Company with a request:

          (a)  to register their transfer; or

          (b)  to exchange such Warrant for an equal number of warrants of other
authorized denominations,

the Company shall register the transfer or make the exchange as requested if the
following requirements are met:

          (x)  the Warrant shall be duly endorsed or accompanied by a written
instruction of transfer in form satisfactory to the Company, duly executed by
the Holder thereof or by his attorney-in-fact, duly authorized in writing; and

          (y)  in the case of Shares, such request shall be accompanied by the
following additional information and documents (all of which may be submitted by
facsimile), as applicable:

               (i)    if such Shares are being transferred (1) to a "qualified
     institutional buyer" (as defined in Rule 144A) in accordance with Rule 144A
     or (2) pursuant to an exemption from registration in accordance with Rule
     144 (and based on an opinion of counsel if the Company so requests) or (3)
     pursuant to an effective registration statement under the Securities Act, a
     certification to that effect;

               (ii)   if such Shares are being transferred pursuant to an
     exemption from registration in accordance with Rule 904 under the
     Securities Act (and based on an opinion of counsel if the Company so
     requests), a certification to that effect; or

               (iii)  if such Shares are being transferred in reliance on
     another exemption from the registration requirements of the Securities Act
     (and based on an opinion of counsel if the Company so requests), a
     certification to that effect.

     7.   Representations and Warranties. The Company represents and warrants to
the Holder as follows:

                                       11
<PAGE>

          (a)  Organization and Powers.  The Company (i) is a corporation duly
organized, validly existing and in good standing under the laws of the
jurisdiction of its incorporation; and (ii) has all the requisite power and
authority to carry on its business and to execute, deliver and perform its
obligations under this Warrant.

          (b)  Authorization; No Conflict. The offer and sale of the Warrant and
the Common Stock underlying the Warrant, and the execution, delivery and
performance by the Company of the Warrant have been duly authorized by all
necessary corporate action of the Company and do not and will not (i) contravene
the Company's articles of incorporation or bylaws; (ii) result in a breach or
default under any material instrument, contract or other agreement to which the
Company is a party; or (iii) violate any provision of any law, rule, regulation,
order, judgment, decree or the like binding on or affecting the Company.

          (c)  Binding Obligations. The Warrant constitutes, or will constitute,
a legal, valid and binding obligation of the Company, enforceable against the
Company in accordance with its terms, except as may be limited by bankruptcy,
insolvency, reorganization, moratorium or other similar laws relating to or
affecting rights of creditors and other obligees generally and general
principles of equity, including, without limitation, concepts of materiality,
reasonableness, good faith and fair dealing and the possible unavailability of
specific performance or injunctive relief, regardless of whether considered in a
proceeding of equity or at law.

          (d)  Shares Duly Issued, Fully Paid and Non-Assessable.  The Shares,
when issued upon the exercise of this Warrant pursuant to the terms hereof,
shall be duly issued, fully paid and non-assessable.

          (e)  No Registration.  The offer and sale to the Holder of the Warrant
and the issuance to the Holder of underlying Shares of Common Stock upon
exercise of the Warrant are exempt from the registration and prospectus delivery
requirements of the Securities Act.

     8.   Registration Rights.  The Holder shall have such registration rights
with respect to the Shares issuable upon the exercise hereof as specified in
that certain Registration Rights Agreement dated as of date hereof.

  9.      Press Releases.  The Holder shall consent to the form and content of
all press releases or public announcements that shall be made concerning this
Warrant and the transactions contemplated hereby, and the Company shall not make
any press release or public announcement without the Holder's prior written
consent, which consent shall not be unreasonably withheld.

     10.  Miscellaneous.

          (a)  The terms of this Warrant shall be binding upon and shall inure
to the benefit of any successors or assigns of the Company and of the holder or
holders hereof and of the Common Stock issued or issuable upon the exercise
hereof.

                                       12
<PAGE>

          (b)  No Holder, as such, shall be entitled to vote or receive
dividends or be deemed to be a stockholder of the Company for any purpose, nor
shall anything contained in this Warrant be construed (i) to confer upon the
Holder, as such, any rights of a stockholder of the Company, or any right to
vote, give or withhold consent to any corporate action, receive notice of
meetings, receive dividends or subscription rights, or otherwise, or (ii) as
imposing any obligation on the Holder to purchase any securities or any
liability as a stockholder of the Company, whether such obligation or
liabilities are asserted by the Company or its creditors.

          (c)  Receipt of this Warrant by the Holder hereof shall constitute
acceptance of and agreement to the foregoing terms and conditions.

          (d)  The Company will not, by amendment of its certificate of
incorporation or bylaws or through any other action, avoid or seek to avoid the
observance or performance of any of the terms of this Warrant, but will at all
times in good faith assist in the carrying out of all such terms and in the
taking of all actions as may be necessary or appropriate in order to protect the
rights of the Holder against impairment.

          (e)  Upon receipt of evidence reasonably satisfactory to the Company
of the loss, theft, destruction or mutilation of this Warrant and, in the case
of any such loss, theft or distribution, upon delivery of an indemnity agreement
reasonably satisfactory in form and amount to the Company or, in the case of any
such mutilation, upon surrender and cancellation of such Warrants, the Company
at its expense will execute and deliver, in lieu thereof, a new Warrant of like
date and tenor.

          (f)  Any provision of this Warrant may be amended, waived or modified
upon the written consent of the Company and the Holder.

          (g)  The Company hereby agrees that at all times there shall be
reserved for issuance and/or deliver upon exercise of this Warrant, free from
preemptive rights, such number of authorized but unissued shares of Common Stock
as from time to time shall be required for issuance or delivery upon exercise of
this Warrant. The Company further agrees that it will promptly to take all
action as may from time to time be required in order to permit the holder hereof
to exercise this Warrant and the Company duly and effectively to issue shares of
Common Stock hereunder.

          (h)  Any Warrants issued hereby shall be deemed to be milestone
adjustments under the Internet and Portal Agreement.

          (i)  This Warrant shall be governed by and construed in accordance
with the internal laws of the State of Delaware without regard to the conflicts
of laws provisions thereof.


               [Remainder of this page intentionally left blank.]

                                       13
<PAGE>

     IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by its
duly authorized officer.

Dated: April __, 2000


                                                      CAIS INTERNET, INC.




                                                      --------------------------
                                                      William M. Caldwell, IV
                                                      President

                                      S-1
<PAGE>

                                   EXHIBIT A
                                   ---------
                               NOTICE OF EXERCISE

TO: CAIS Internet, Inc.

     1.   The undersigned hereby elects to purchase _______________ shares of
the Common Stock of CAIS Internet, Inc. pursuant to the terms of the attached
Warrant, and tenders herewith payment of the purchase price of such shares in
full, together with all applicable transfer taxes, if any.

     2.   The undersigned hereby elects to exercise the purchase right with
respect to ___________ shares of such Common Stock through Cashless Exercise, as
set forth in Section 3 of the attached Warrant.

     3.   Please issue a certificate or certificates representing said shares of
Common Stock in the name of the undersigned or in such other names as is
specified below:

          __________________________________________
          (Name)

          __________________________________________
          __________________________________________
          (Address)


                                    Signature of Holder:


                                    By:_______________________________

                                    Title:____________________________

                                    Date:_____________________________