# Cisco Networking/CCENT/Subnetting

This lesson covers subnetting.

## Objectives and Skills

Objectives and skills for the subnetting portion of Cisco CCENT certification include:
* Identify the appropriate IPv4 addressing scheme using VLSM and summarization to satisfy addressing requirements in a LAN/WAN environment

## Readings

* Wikipedia: Subnetwork
* Wikipedia: IPv4 subnetting reference
* Wikipedia: Classless Inter-Domain Routing
* Wikipedia: Supernetwork
* Cisco: IP Addressing and Subnetting for New Users

## Multimedia

* YouTube: Binary Math - CompTIA Network+ N10-005: 1.3
* YouTube: Subnetting - CompTIA Network+ N10-005: 1.3
* YouTube: Subnetting, Cisco CCNA, Binary Numbers - Part 1
* YouTube: Subnetting, Cisco CCNA, Binary Numbers - Part 2
* YouTube: Subnetting, Cisco CCNA, Binary Numbers - Part 3
* YouTube: Subnetting, Cisco CCNA, Binary Numbers - Part 4
* YouTube: Subnetting Cisco CCNA - Part 1 The Magic Number
* YouTube: Subnetting Cisco CCNA - Part 2 The Magic Number
* YouTube: Subnetting Cisco CCNA - Part 3 The Magic Number
* YouTube: Subnetting Cisco CCNA - Part 4 The Magic Number
* YouTube: Subnetting Cisco CCNA - Part 5 The Magic Number
* YouTube: Subnetting Cisco CCNA - Part 6 The Magic Number

## Activities

* Review 3com: Understanding IP Addressing: Everything You Ever Wanted To Know. Complete all exercises in Appendix B (page 57).
* Review EasySubnetting.com subnetting resources and complete multiple subnetting exercises.
* Generate practice subnetting questions using the TunnelsUp: Subnet Calculator.
* Play the Cisco: Subnet Troubleshooting Game and practice until you can consistently achieve a high score.
* Play the Subnetting.net Subnetting Game and practice until you can consistently achieve a high score.
* Play the Insite: Cisco Subnet Slingshot Game and practice until you can consistently achieve a high score.
* Review Subnet Ninja: Subnetting How To Guide and verify your answers with the Subnet Calculator
* Check your Subnets and Masks Online with this Subnetting Calculator and verify that your subnet masks and CIDR is correct.

## Lesson Summary

* A subnetwork, or subnet, is a logical, visible subdivision of an IP network.
* The practice of dividing a network into two or more networks is called subnetting.
* An IP address has two fields, a network prefix and a host identifier.
* The network prefix is identified using CIDR notation.
* In IPv4, the network prefix may also be identified using a 32-bit subnet mask in dotted-decimal notation.
* A network is divided into two or more subnetworks by dividing the host identifier field into separate subnet number and host identifier fields.
* All hosts on a subnetwork have the same network prefix.
* Traffic between subnets is exchanged through a router.
* The first address on any given IPv4 network or subnet is reserved for the network itself.
* The last address on any given IPv4 network or subnet is reserved for broadcast.
* The separation of the network prefix/subnet number from the host identifier is performed by a bitwise AND operation between the IP address and the (sub)network mask.
* The number of subnetworks created by subnetting can be calculated as 2$^n$, where n is the number of bits used for subnetting.
* The number of available hosts on each subnet can be calculated as 2$^{n-2}$, where n is the number of bits available for the host identifier.
* Traditionally, the first network, known as subnet zero, and the last network, known as the all-ones subnet, were not used on production networks. This practice was declared obsolete by RFC 1878 in 1995.
* The goal of Classless Inter-Domain Routing was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.
* Classless Inter-Domain Routing is based on variable-length subnet masking (VLSM), which allows a network to be divided into variously sized subnets, providing the opportunity to size a network more appropriately for local needs.
* The benefits of supernetting are conservation of address space and efficiencies gained in routers in terms of memory storage of route information and processing overhead when matching routes.

## Key Terms
### binary mask

    Data that is used for bitwise operations to set multiple bits either on, off or inverted in a single bitwise operation.

### bitwise AND

    A binary operation that takes two representations of equal length and performs the logical AND operation on each pair of corresponding bits. The result in each position is 1 if the first bit is 1 and the second bit is 1; otherwise, the result is 0.

### broadcast address

    A logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams. In IPv4 networks, the broadcast address is the all-ones address, the last address on the network subnet.

### network address

    The address of a network or subnetwork. In IPv4 networks. the network address is the all-zeros address, the first address on the network subnet.

### prefix mask

    A subnet mask specified in CIDR notation.

### provider-independent address space

    A block of IP addresses assigned by a regional Internet registry (RIR) directly to an end-user organization.

### routing table

    A data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes.

### subnet

    A logical, visible subdivision of an IP network.

### subnet address

    A logically visible subdivision of an IP network.

### subnet mask

    A bitmask that encodes the (sub)network prefix length in dotted-decimal notation, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format.

### subnetting

    The practice of dividing a network into two or more networks.

### supernet

    An Internet Protocol (IP) network that is formed from the combination of two or more networks (or subnets) with a common Classless Inter-Domain Routing (CIDR) prefix.

### variable-length subnet masks (VLSM)

    Used to divide a network into variously sized subnets, as opposed to fixed-length subnet masks used in classful addressing.

## Review Questions

* A subnetwork, or subnet, is _____. A subnetwork, or subnet, is a logical, visible subdivision of an IP network.
* The practice of dividing a network into two or more networks is called _____. The practice of dividing a network into two or more networks is called subnetting.
* An IP address has two fields, _____ and _____. An IP address has two fields, a network prefix and a host identifier.
* The network prefix is identified using _____. The network prefix is identified using CIDR notation.
* In IPv4, the network prefix may also be identified using _____. In IPv4, the network prefix may also be identified using a 32-bit subnet mask in dotted-decimal notation.
* A network is divided into two or more subnetworks by _____. A network is divided into two or more subnetworks by dividing the host identifier field into separate subnet number and host identifier fields.
* All hosts on a subnetwork have _____. All hosts on a subnetwork have the same network prefix.
* Traffic between subnets is exchanged _____. Traffic between subnets is exchanged through a router.
* The first address on any given IPv4 network or subnet is _____. The first address on any given IPv4 network or subnet is reserved for the network itself.
* The last address on any given IPv4 network or subnet is _____. The last address on any given IPv4 network or subnet is reserved for broadcast.
* The separation of the network prefix/subnet number from the host identifier is performed by _____. The separation of the network prefix/subnet number from the host identifier is performed by a bitwise AND operation between the IP address and the (sub)network mask.
* The number of subnetworks created by subnetting can be calculated as _____. The number of subnetworks created by subnetting can be calculated as 2n, where n is the number of bits used for subnetting.
* The number of available hosts on each subnet can be calculated as _____. The number of available hosts on each subnet can be calculated as 2n-2, where n is the number of bits available for the host identifier.
* Traditionally, the first network, known as _____, and the last network, known as _____, were not used on production networks. This practice was _____. Traditionally, the first network, known as subnet zero, and the last network, known as the all-ones subnet, were not used on production networks. This practice was declared obsolete by RFC 1878 in 1995.
* The goal of Classless Inter-Domain Routing was to _____. The goal of Classless Inter-Domain Routing was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.
* Classless Inter-Domain Routing is based on _____. Classless Inter-Domain Routing is based on variable-length subnet masking (VLSM), which allows a network to be divided into variously sized subnets, providing the opportunity to size a network more appropriately for local needs.
* The benefits of supernetting are _____. The benefits of supernetting are conservation of address space and efficiencies gained in routers in terms of memory storage of route information and processing overhead when matching routes.

## Assessments

* Flashcards: Quizlet: CCENT - Subnetting
* Flashcards: Quizlet: Subnet Mask - Mask Bits
* Flashcards: Quizlet: Subnetting
* Quiz: Quizlet: CCENT - Subnetting
* Quiz: Quizlet: Subnet Mask - Mask Bits
* Quiz: Quizlet: Subnetting

## See Also

* Internet Protocol Analysis/Subnetting