Chapter 3. Extending IP Addresses

Terms you'll need to understand:

• CIDR

• IETF

• ip helper

• ip subnet-zero

• IPv6

• Prefix length

• Route summarization

• Supernetting

• Variable-length subnet masking (VLSM)

Techniques you'll need to master:

• Reviewing the IP subnetting process

• Understanding the formulas for calculating subnets and hosts

• Implementing CIDR addressing or supernetting

• Expressing various networks and subnets with their CIDR representations

• Using VLSM to extend an IP address scheme

• Deploying route summarization techniques

• Configuring the ip helper command for controlled broadcasts

The Internet Engineering Task Force (IETF) is a governing body that consists of more than 80 groups working together to develop Internet standards. The Internet Protocol (IP) is the primary Layer 3 protocol used to encapsulate data in the Internet suite. In addition to being routable, IP provides error coverage, fragmentation, and reassembly of datagrams for transmission over networks with different maximum data unit sizes. The IETF first defined a globally unique, 32-bit number for IP addressing in 1981. These globally unique logical addresses enable IP networks to communicate with each other from anywhere in the world.

Due to the global and somewhat random allocation of this finite pool of addresses (2 ³² ), the Network Information Center (InterNIC) has run out of address space. In addition, approximately 5,000 routes were using the Internet in 1990. By the end of the millennium , more than 72,000 routes existed on the Internet, and today there are easily more than 100,000. The sizes of routing tables have been growing seemingly exponentially. This chapter focuses on some of the solutions and mechanisms the Cisco professional is expected to know to help decrease the routing table size as well as create a more hierarchical addressing scheme.