<PAGE>

                                                                   Exhibit 10.18

                              SUBLEASE AGREEMENT
                              ------------------


     THIS SUBLEASE AGREEMENT (the "Sublease") is made as of November 23/rd/,
 1999 (the "Sublease Date") by and between ANTEC CORPORATION, a Delaware
 corporation ("Tenant") and nFRONT, INC., a Georgia corporation ("Subtenant")
 (the words "Tenant" and "Subtenant" to include their respective legal
 representatives, successors and permitted assigns where the context requires or
 permits).

                                    RECITALS

     A.  New York Life Insurance Company ("Prior Landlord") and Tenant entered
into that certain Lease Agreement dated August 11, 1994 for approximately 58,672
square feet of space at 5720 Peachtree Parkway, Norcross, Gwinnett County,
Georgia (the "Demised Premises"), as amended by First Amendment to Lease dated
December 12, 1994 between Prior Landlord and Tenant (the Lease, as amended by
herein referred to as the "Lease").

     B.  Prior Landlord has conveyed its interest in the Demised Premises and
the Lease to WKK Atlanta Investments, L.P. ("Landlord").

     C.  Tenant desires to sublease the Demised Premises to Subtenant.

     D.  Tenant and Subtenant desire to set forth their agreement upon the
following terms and conditions.

                                   AGREEMENT

     NOW, THEREFORE, in consideration of the premises, Ten and No/100 Dollars
($10.00) in hand paid by each party to the other, the mutual covenants contained
herein, and other good and valuable consideration, the receipt and sufficiency
of which are hereby acknowledged, the parties hereto agree to the following (all
capitalized terms not defined in this Sublease shall have the meaning given to
such terms in the Lease):

     1.  Term. Tenant does hereby lease and demise unto Subtenant, and Subtenant
         ----
does hereby hire, lease and accept from Tenant the Demised Premises for a term
(the "Term") commencing on the Sublease Date ("Sublease Commencement Date") and
continuing until July 5, 2005 ("Expiration Date"). For purposes of this Sublease
Agreement, the Demised Premises shall include all existing fixtures, equipment
and improvements in the Demised Premises, including without limitation the
security system, the CAT 5 voice and data cabling now located and existing in
the Demised Premises.

     2.  Rent. Subtenant shall pay to Tenant base rent for the Demised Premises
         ----
in the following amounts on a monthly basis (the "Sublease Base Rent"):
<PAGE>

         Sublease Date - Rent Start Date:         $ - 0 -
         Rent Start Date - October 31, 2000:      $67,228.33
         November 1, 2000 - October 31, 2001:     $68,572.90
         November 1, 2001 - October 31, 2002:     $69,944.36
         November 1, 2002 - October 31, 2003:     $71,343.24
         November 1, 2003 - October 31, 2004:     $72,770.11
         November 1, 2004 - July 5, 2005:         $74,225.53

Each payment shall be made on the first day of each month during the Term,
beginning on the Rent Start Date, payable in advance, without demand and without
abatement, reduction, set-off or deduction, with appropriate prorations for
partial months. The "Rent Start Date" shall be the later of December 1, 1999, or
the thirtieth (30th) day following Landlord's consent to this Sublease. If such
consent is obtained after November 1, 1999, then for each day thereafter until
the consent is obtained the above schedule shall be delayed by the number of
days that occur until the consent is obtained. For purposes of this Section,
"Landlord's consent" shall be deemed to mean the date of Landlord's execution
and delivery to Subtenant of the "Consent to Sublease" in the form of Exhibit C
                                                                      ---------
attached hereto and made a part hereof.

In addition to Sublease Base Rent, Subtenant agrees to pay to Tenant Sublease
Maintenance Rent (as defined below) beginning on January 1, 2001 and continuing
thereafter until the Expiration Date. "Sublease Maintenance Rent" shall be the
excess of the Operating Expenses for each calendar year ("Year") over the
greater of (a) Operating Expenses for Year 2000, or (b) $293,360.00. Sublease
Maintenance Rent for a given Year shall be paid by Subtenant to Tenant on a
equal monthly basis, along with and at the same place and manner as provided
herein for Sublease Base Rent. Subject to Section 5 of the Special Stipulations
attached hereto as Exhibit B, the Sublease Maintenance Rent shall be calculated
                   ---------
as follows: Sublease Maintenance Rent for each Year beginning on January 1, 2001
shall be paid based on the difference between Landlord's Projection of Operating
Expenses for that Year and the greater of (a) Actual Operating Expenses for Year
2000 as determined by the Lease, or (b) $293,360.00 (Tenant hereby agreeing to
deliver such notice of Landlord's Projection of Operating Expenses and the
Actual Operating Expenses to Subtenant promptly after its receipt of such
notice). For Year 2001 only, Subtenant's payments of Sublease Maintenance Rent
shall be based on the Landlord's Projection of Operating Expenses for Years 2000
and 2001 until the Actual Operating Expenses for Year 2000 are available and
delivered to Tenant and Subtenant pursuant to the Lease, and the remaining
installments of Sublease Maintenance Rent due for Year 2001 shall be adjusted to
account for any overpayments or underpayments by Subtenant for such Year. As
Tenant is notified by Landlord of the Actual Operating Expenses for a Year,
Tenant shall forward such information to Subtenant. Within thirty (30) days
after receipt of such statement, subject to Section 5 of the Special
Stipulations attached hereto as Exhibit B, Subtenant shall pay to Tenant any
deficiency between the Actual Operating Expenses for that Year and the amount of
Sublease Maintenance Rent paid by Subtenant, or in the event of overpayment,
Tenant shall credit the amount of such overpayment toward the next installment
of Sublease Maintenance Rent owed by Tenant or remit such overpayment to
Subtenant if the Term has expired or has been terminated and no default exists
hereunder. The obligations in the immediately preceding sentence shall survive
the expiration or any earlier termination of this Sublease. If Subtenant shall
elect to exercise the right to audit the Landlord's books and records relating
to Operating Costs as allowed in the Lease, and only to the extent such right
exists, Tenant shall cooperate with Subtenant to enforce such rights provided
Subtenant shall pay all costs and expenses therefore. If as a result of such
audit or otherwise it is determined that Landlord has collected or charged
Tenant or Subtenant more than Landlord is entitled to collect, Tenant shall
refund such

                                       2
<PAGE>

overpayments to Subtenant only when, to the same extent, and on the same basis
as Tenant receives a refund of such overpayments from Landlord.

All payment of Sublease Base Rent and Sublease Maintenance Rent shall be paid to
ANTEC Corporation, 11450 Technology Circle, Duluth, Georgia 30097, Attention:
Mr. Deon Sanders. By written notice, Landlord may direct Subtenant to deliver
rent checks to a different address.

In addition to Sublease Base Rent and Sublease Maintenance Rent, Subtenant
agrees to pay to Tenant all Additional Rent (as that term is defined in Section
1(g) of the Lease) and otherwise perform all tasks required of Tenant under
Section 1(g) of the Lease upon notice from Tenant. Tenant represents to
Subtenant that Tenant does not own any personal property that is located on the
Demised Premises.

     3.   Landlord Obligations. With respect to payments, consents, work,
          --------------------
services or the performance of any and all other obligations required of
Landlord under the Lease, Tenant's sole obligation to Subtenant with respect
thereto shall be to request the same, upon request in writing by Subtenant, and
to use reasonable efforts to obtain the payment, consent or other performance by
Landlord; provided, however, the foregoing shall not require Tenant to institute
          -----------------
any legal action to obtain such performance unless Subtenant shall indemnify and
hold harmless Tenant against all costs, including reasonable attorneys' fees of
such enforcement. Subtenant shall have the right to request the same directly
from Landlord, and to conduct such proceedings as may be required to obtain from
Landlord any such work and services or the performance of such obligations.
Tenant agrees to cooperate with Subtenant in connection therewith and to execute
such documents as may be reasonably required in connection therewith.

     4.   Consent. In all cases where consent of Landlord is required under the
          -------
Lease, Subtenant shall be required to obtain the consent of Tenant (which
consent Tenant agrees shall not be unreasonably withheld) and Landlord. It shall
be a condition to the obligations of Tenant and Subtenant hereunder that
Landlord and, to the extent required, Landlord's mortgagee consent to this
Sublease in the form attached hereto as Exhibit C and made a part hereof on or
                                        ---------
before December 1, 1999. If such consent has not be obtained on or before such
date, then thereafter either party may terminate this Sublease Agreement by
written notice to the other party, in which event the parties shall be released
and relieved of all further rights, liabilities and obligations to each other
hereunder.

     5.   Relationship to Lease.
          ---------------------

          (a) This Sublease and all of Subtenant's rights hereunder are
expressly subject and subordinate to all of the terms of the Lease, a copy of
which is attached hereto and incorporated herein as Exhibit A. Subtenant hereby
                                                    ---------
agrees to assume all obligations of and restrictions imposed on Tenant under the
Lease with respect to the Demised Premises, except for Tenant's obligations to
pay Base Rent and Maintenance Rent to Landlord under the Lease. Subtenant shall
and hereby does indemnify Tenant and hold Tenant harmless from and against any
and all expense, loss, and liability suffered by Tenant (except to the extent
that such expenses, losses, and liabilities arise out of Tenant's own negligence
or willful act), by reason of Subtenant's breach of any of the provisions of
this Section 5. Subtenant hereby acknowledges that Subtenant shall look solely
to Landlord for the performance of all the Landlord's obligations under the
Lease and that Tenant shall not be obligated to provide any services to
Subtenant or otherwise perform any obligations in connection with this Sublease,
except to the extent enforcement of such rights requires the reasonable
assistance and/or cooperation of Tenant.

                                       3
<PAGE>

Subtenant acknowledges that any termination of the Lease will result in a
termination of the Sublease.

          (b) Provided Subtenant is not in default hereunder, Tenant shall pay
the Base Rent, Maintenance Rent due to Landlord under the Lease so as not to
cause a default under the Lease. Tenant shall indemnify and hold harmless
Subtenant against all damage, liability, losses, costs and expenses resulting
to, or that are incurred by Subtenant relating to Tenant's failure to pay any of
the Base Rent or Maintenance Rent due the Landlord under the Lease. If Tenant
shall be in material default of a monetary nature or fail to pay Base Rent or
Maintenance Rent due to Landlord under the Lease, Subtenant shall have the right
to offset against its obligations hereunder and pay such Base Rent or
Maintenance Rent directly to Landlord as long as Landlord agrees to accept such
payments from Subtenant, provided in all such events Tenant shall remain
obligated to pay to Subtenant the difference between the Base Rent or
Maintenance Rent due to Landlord under the Lease and the Base Rent or
Maintenance Rent due to Tenant under this Sublease.

     6.   Use. Subtenant shall use the Demised Premises as general office
          ---
purposes and for no other purpose. Tenant makes no (and does hereby expressly
disclaim any) covenant, representation or warranty as to the use being allowed
by or being in compliance with any applicable laws, rules, ordinances or
restrictive covenants now or hereafter affecting the Demised Premises.
Notwithstanding the foregoing, Tenant represents that during its occupancy of
the Demised Premises and to the best of its knowledge, Tenant received no
written notice of any violation of zoning in respect to the use of the Demised
Premises by Tenant.

     7.   Default. Any act or omission by Subtenant that would constitute a
          -------
default under the Lease shall, subject to the same notice and cure provisions
provided in the Lease, if any, be deemed a default by Subtenant under this
Sublease (except for payment of Base Rent and Maintenance Rent to Landlord under
the Lease, which is the obligation of Tenant). In addition, any failure by
Subtenant to pay Sublease Base Rent or Sublease Maintenance Rent when due, any
failure by Subtenant to pay Additional Rent under the Lease, or any failure by
Subtenant to perform any other obligations required under this Sublease, shall
be deemed a default hereunder, provided that Subtenant shall be entitled to
receive the same notices and have the same cure rights that Tenant has under the
Lease. Any such default by Subtenant shall entitle Tenant to exercise any and
all remedies available to Landlord under the Lease or any other remedies
available at law or in equity under the laws of the State of Georgia.

     8.   Assignment of Tenant's Right against Subtenant. Tenant and Subtenant
          ----------------------------------------------
hereby agree that, if Subtenant shall be in default of any obligation of
Subtenant under the Sublease, which default also constitutes a default by Tenant
under the Lease, then Landlord shall be permitted to avail itself of all the
rights and remedies available to Tenant in connection therewith.

     9.   Quiet Enjoyment. Provided Subtenant has performed its obligations
          ---------------
hereunder, Subtenant shall have the quiet enjoyment of the Demised Premises
during the Term without interference by Tenant or anyone claiming by, through or
under Tenant.

     10.  Insurance and Indemnities. Subtenant hereby agrees to indemnify and
          -------------------------
hold Landlord and Tenant harmless, with regard to Subtenant's leasing and use of
the Demised Premises, to the same extent that Tenant is required to indemnify
and hold Landlord harmless with respect to the Demised Premises under the Lease.
Likewise, Subtenant hereby agrees to

                                       4
<PAGE>

obtain and provide evidence satisfactory to Tenant that Subtenant is carrying
insurance in the same amounts and of the same types required to be carried by
Tenant under the Lease with regard to the Demised Premises, and Subtenant shall
name Tenant as additional insured on all such policies of insurance. Tenant
hereby agrees to indemnify and hold Subtenant harmless from costs, losses, and
damages, resulting from claims Landlord asserts under the Lease that are related
solely to periods prior to the Sublease Date.

     11.  Subleasing and Assignment. Except to the extent allowed by the Lease,
          -------------------------
Subtenant shall have no right to sublease or assign its rights under this
Sublease or its rights with regard to the Demised Premises without the prior
written consent of Tenant and Landlord, which consent may be withheld in
Tenant's or Landlord's sole discretion (Tenant's approval shall not be
unreasonably withheld, delayed, or denied; Landlord's approval shall be as
required by the Lease).

     12.  Condition of Demised Premises. Upon the expiration or earlier
          -----------------------------
termination of this Sublease, Subtenant shall return the Demised Premises to
Tenant in the condition required by the Lease.

     13.  Miscellaneous. This Sublease shall be governed by the laws of the
          -------------
State of Georgia. Time shall be of the essence with regard to the obligations
under this Sublease. This Sublease supersedes all prior discussions and
agreements between the parties and incorporates their entire Agreement. Any
capitalized term used in this Sublease which is not defined herein shall have
the same meaning attributable to that term in the Lease. Nothing contained in
this Sublease shall serve to release Tenant from the further performance by
Tenant of Tenant's obligations under the Lease or to relieve Tenant from any of
its liability under the Lease. This Sublease is further subject to the Special
Stipulations contained on Exhibit B, attached hereto and made a part hereof.
                          ---------

     14.  Tenant Warranties Regarding the Lease. Tenant hereby warrants and
          -------------------------------------
represents that (a) the Lease is in full force and effect, has not been modified
or amended except as provided herein, (b) to the best of its knowledge, Tenant
is not in default thereunder; (c) subject only to obtaining the consent of
Landlord, Tenant is authorized and has the power to enter into this Sublease and
to lease the Subleased Premises to Tenant pursuant to the Terms hereof and (d)
the Expiration of the Term of the Lease does not occur earlier than July 5,
2005. During the Sublease Term, Tenant agrees not to modify the terms of the
Lease without Subtenant's consent, which shall not be unreasonably withheld or
delayed.

     15.  Notices. Notices to be given under this Sublease shall be given in the
          -------
same manner and pursuant to the same terms as contained in the Lease. Promptly
upon its receipt of any notice given under the Lease, Tenant shall provide such
notices to Subtenant. Notices shall be given to:

          If to Tenant:

          ANTEC Corporation
          11450 Technology Circle
          Duluth, Georgia 30097
          Attn:  Mr. Paul Barilla

                                       5
<PAGE>

          With a copy to:

          ANTEC Corporation
          11450 Technology Circle
          Duluth, Georgia 30097
          Attn:  Mr. Mark Scagliuso

          If to Subtenant until the Rent Start Date:

          nFront, Inc.
          520 Guthridge Court, NW
          Suite 100
          Norcross, Georgia 30092
          Attn: Chief Financial Officer

          If to Subtenant after the date Subtenant commences operations in the
          Demised Premises (based on written notice to Tenant):

          nFront, Inc.
          5720 Peachtree Parkway
          Norcross, Georgia 30092
          Attn:  Chief Financial Officer

     16.  Subordination, Non-Disturbance and Attornment. Tenant agrees to
          ---------------------------------------------
cooperate and assist Subtenant in its efforts to obtain a Subordination, Non-
Disturbance and Attornment Agreement from Landlord and Landlord's lender
consistent with and pursuant to the terms of the Lease.

                           [SIGNATURES ON NEXT PAGE)

                                       6
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have hereunto set their hands under
seals, the day and year first above written.


                                 TENANT:
                                 ------

                                 ANTEC CORPORATION, a Delaware
                                 corporation


                               By: /s/ Mark S. Scagliuso
                                  -----------------------------------
                                   Name: Mark S. Scagliuso
                                   Title: C.A.O.


                               Attest: /S/ Paul Bonilla
                                      ----------------------------------
                                       Name: Paul Bonilla
                                       Attest:


                                               [CORPORATE SEAL]


                               SUBTENANT:

                               nFRONT, INC., a Georgia corporation

                               By: /s/ Jeff Hodges
                                  --------------------------------------
                                   Name: Jeff Hodges
                                   Title: CFO


                               Attest: /s/ Craig L. Benn
                                      ----------------------------------
                                       Name:  Craig L. Benn
                                       Attest: Vice President Controller

                                                [CORPORATE SEAL]

                                       7
<PAGE>

                               LIST OF EXHIBITS
                               ----------------

                    Exhibit A          Underlying Lease

                    Exhibit B          Special Stipulations

                    Exhibit C          Consent to Sublease
<PAGE>

                          FIRST AMENDMENT TO SUBLEASE


     THIS FIRST AMENDMENT is entered into as of the 1/st/ day of February 2000,
     by and between Antec Corporation, a Delaware Corporation (Tenant) and
     nFront, Inc., a Georgia Corporation (Subtenant).


     A.  New York Life Insurance Company ("Prior Landlord") and Tenant entered
into that certain Lease Agreement dated August 11, 1994 for approximately 58762
square feet of space of 5720 Peachtree Parkway, Norcross, Gwinnett county,
Georgia (the "Demised Premises") as amended by First Amendment to Lease dated
December 12, 1994 between Prior Landlord and Tenant.

     B.  Prior Landlord has conveyed its interest in the Demised Premises and
the Lease To WKK Atlanta Investments, L.P. ("Landlord").

     C.  Tenant and Subtenant entered into a sublease agreement dated November
23, 1999 for the Demised Premises (the "Sublease").

     D.  LandLord's consent to the Sublease was received on November 23, 1999.

     NOW THEREFORE in consideration of the mutual promises and for other good
and valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the Tenant and Subtenant agree as follows:

     1.  The Sublease is hereby amended by deleting Special Stipulation 3 on
Exhibit B to the sublease in its entirety and the following is substituted
therefore:

         "3. Parking. Within one hundred and twenty days (120) after the
             -------
     Sublease Date (as extended for such additional time as is equal to the time
     lost by Tenant or Tenant's contractors or suppliers in connection with the
     construction of the parking area due to floods or weather conditions
     actually and materially delaying such construction). Tenant shall make
     available to Subtenant employees, customers, and visitors at no cost to
     Subtenant, a minimum of 243 automobile parking spaces (on an unassigned
     basis) in the parking areas of the Demised Premises as shown on Attachment
     1, attached hereto and made a part hereof.

     2.  Attachment 1 has been modified to reflect the relocation of parking
lot light poles and the increase in number of such light poles from 5 to 6, as
requested by a n-Front, inc. n-Front agrees to reimburse Antec Corporation for
the cost of this incremental work not to exceed $12,000.00

     3.  Except as amended hereby, the Sublease shall be and remain in full
force and effect and unchanged. The Sublease is hereby ratified and confirmed by
Tenant and Subtenant To the extent the terms hereof are inconsistent with the
terms of the Sublease, the terms hereof shall control.

Antec Corporation                            n-Front, Inc.


By:      /s/ A. Rois-Mendez                  By:   /s/ Jeffrey W. Hodges
      ------------------------                     ------------------------
           A. Rois-Mendez                             Jeffrey W. Hodges
      ------------------------                     ------------------------
Title: Senior Counsel and                    Title: CFC
      ------------------------                     ------------------------
       Assistant Secretary
      ------------------------
<PAGE>

     (h)  All sums payable pursuant to the provisions of this Section 1 by
Tenant, or which are at the expense of Tenant, are deemed and considered to be
Rent hereunder, and, if not paid, Landlord shall have all rights and remedies
provided in this Lease, at law or in equity for the non-payment of Rent. All
Rent payable pursuant to the terms of this Section 1 by Tenant to Landlord shall
be paid promptly as and when the same shall become due and payable, without
notice, demand, abatement, deduction or set-off, and shall be paid to such
person or persons and at such locations or addresses as Landlord may from time
to time designate in writing.

     (i)  Although the cost of janitorial services to the Premises and to the
Building shall initially be included within the Maintenance Rent, Tenant shall
have the right to reduce the degree or quantity of services provided by the
Building's janitorial service or to substitute and pay directly another
janitorial service reasonably approved in advance in writing by Landlord, in
either of which events the Maintenance Rent (including the base amount thereof
and the Operating Expenses attributable to the Premises) shall be adjusted to
give Tenant an equitable credit (in an amount based upon but not to exceed the
per square foot cost of janitorial services then being paid by Landlord for
janitorial services within the Building) by virtue of the fact that Tenant makes
such a reduced or direct payment of a portion thereof for all or some of the
janitorial services which would otherwise be included in its Maintenance Rent
and Operating Expenses.

     2.   SERVICES; WORK

          As long as Tenant is not in default following receipt of notice (if
required under this Lease) and the expiration of the cure period, if any, under
any of the covenants or provisions of this Lease, Landlord shall provide the
following services