008 - Networking - C5.1.5 Variable Length Subnet Mask (VLSM) Facts

Classful IP Addresses

Classful addresses are IP addresses that use a default subnet mask, as follows: -• Class A: 255.0.0.0 -• Class B: 255.255.0.0 -• Class C: 255.255.255.0 They are considered classful because default subnet mask identifies network portion and host portion of IP address.

Classless IP Addresses

Classless addresses, on the other hand, use custom mask value to separate network and host portions of IP address. Classless addressing is made possible using Classless Inter-Domain Routing (CIDR). It allows you to use only part of octet for network address. This called partial subnetting, or variable-length subnet masking (VLSM).

VLSM - Variable-length subnet masking.

When using VLSM, you ignore default subnet mask boundaries and specify custom number of subnet mask bits. Example - you could define subnet of 255.255.252.0. In addition to the first and second octets, this mask also assigns first six bits in the third octet to be used for network portion of address. This mask would appear in binary notation as follows: 11111111.11111111.11111100.00000000 As you can see, six bits are reallocated from host address to network address. This allows you to create additional subnets, but it reduces number of host addresses available within each one.