OSPF and EIGRP

Variable length subnet design can be greatly simplified by following a simple rule. First, subnet the basic network address using the shortest mask you need. Then, take one of these subnets and subnet it again using the larger (more bits) mask.

For example, a corporate network requiring 200 LAN segments and 100 WAN links could be done with a single Class B network by starting with an eight-bit LAN mask of 255.255.255.0. The WAN links only need two hosts per link, so its mask could be 255.255.255.252. For a Class B network address of 144.251.0.0, the LAN subnets could be 144.251.1.0 through 144.251.254.0 (reserving the all-zeros and all-ones subnets).

Take the last two subnets and subnet them again with the longer mask to produce WAN subnets ranging from 144.251.253.0 through 144.251.254.252.

Volume 2 Number 2 Table of Contents