Variable-Length Subnet Masks (VLSMs) allow the use of different masks for the same network number on different Cisco interfaces, which allows the user to conserve IP addresses and more efficiently use available address space.
Enhanced Interior Gateway Router Protocol (EIGRP), Intermediate System-to-Intermediate System (IS-IS), Open Shortest Path First (OSPF), Routing Information Protocol (RIP) Version 2 and Static routes support VLSMs.
Variable-Length Subnet Masking is configured on the router by enabling routing protocols (such as EIGRP or OSPF) that support this, and by configuring the subnet masks of the various Cisco interfaces in the ip address interface sub-command.
The best way to implement VLSMs is to keep the existing numbering plan in place and gradually migrate some networks to VLSMs to recover address space.
In the example below, a 14-bit subnet mask is used, leaving two bits of address space reserved for serial line host addresses.
There is sufficient host address space for two host endpoints on a point-to-point serial link:
| interface ethernet 0 ip address 172.16.1.1 255.255.255.0 !---8 bits of host address space reserved for ethernets |
| interface serial 0 ip address 172.16.254.1 255.255.255.252 !---2 bits of address space reserved for serial lines |
| router ospf 107 !---Router is configured for OSPF and assigned AS 107 |
| network 172.16.0.0 0.0.255.255 area 0.0.0.0 !---Specifies network directly connected to the router |
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