Lecture 6(c): VLSM and Route Summarization

VLSM:Variable Length Subnet Mask. Also called as Subnetting of Subnetting.Subnetting is used to break the network equally.If these networks are subnetted again, different subnets may have different subnet mask values, also called as Variable length subnet mask. With VLSM, IP addressing scheme is used more efficiently without wastage.

we effectively reduce IP's wastege.(See Wan Links of above diagram)

Route Summarization:
Generally Supernetting concept is used in router advertisements, called as route summarization,combines network addresses of same class into one single address to reduce the size of the routing table.It is done on core router to reduce the size of routing table.It is implemented by ISP (internet service providers).

For example, you may want to supernet the networks 192.168.10.0 and 192.168.11.0 to make a single, large network. Following two lines are the conversions of the above network addresses to binaries and the last line is the new subnet mask.

11000000.10101000.0000101 | 0.00000000
11000000.10101000.0000101 | 1.00000000

11111111.11111111.1111111 | 0.00000000

The changed subnet mask is 255.255.254.0 can be used to supernet 192.168.10.0 and 192.168.11.0. The concept of supernetting is used in routing protocols for "route summarization".

Note:Just convert the networks in to binaries, compare the bits of the two networks. Till where you have the similar bit pattern, use a subnet mask bit of "1", and after that "0". Use the altered subnet mask for two networks.