Variable Length Subnet Masks (VLSM)

In 1987, RFC 1009 specified how a subnetted network could use more

than one subnet mask. When an IP network is assigned more than one

subnet mask, it is considered a network with (VLSM) since the

extended network prefixes have different lengths. 

RIP 1 Permits Only a Single Subnet Mask

When using RIP 1, subnet masks have to be uniform across the entire

network prefix. RIP 1 allows only a single subnet mask to be used

within each network number because it does not provide subnet mask

information as part of its routing table update messages. In the absence

of this information, RIP 1 is forced to make assumptions about the mask

that should be applied to any of its learned routes. 

How does a RIP 1 based router know what mask to apply to a route

when it learns a new route from a neighbor? If the router has a subnet

of the same network number assigned to a local interface, it assumes

that the learned subnetwork was defined using the same mask as the

locally configured interface. However, if the router does not have a sub 

net of the learned network number assigned to a local interface, the

router has to assume that the network is not subnetted and applies the

route's natural classful mask. 

For example, assume that Port 1 of a router has been assigned the IP

address 130.24.13.1/24 and that Port 2 has been assigned the IP address

200.14.13.2/24. If the router learns about network 130.24.36.0 from a

neighbor, it applies a /24 mask since Port 1 is configured with another

subnet of the 130.24.0.0 network. However, when the router learns

about network 131.25.0.0 from a neighbor, it assumes a  natural  /16

mask since no other masking information is available. How does a RIP 1

based router know whether it should include the subnet number bits in

a routing table update to a RIP 1 neighbor? A router executing RIP 1

will only advertise the subnet number bits on another port if the update

port is configured with a subnet of the same network number. If the

update port is configured with a different subnet or network number,

the router will only advertise the network portion of the subnet route

and zero out the subnet number field. 

For example, assume that Port 1 of a router has been assigned the IP

address 130.24.13.1/24 and that Port 2 has been assigned the IP address

200.14.13.2/24. Also, assume that the router has learned about network

130.24.36.0 from a neighbor. Since Port 1 is configured with another

subnet of the 130.24.0.0 network, the router assumes that network

130.24.36.0 has a /24 subnet mask. When it comes to advertise this

route, the router advertises 130.24.36.0 on Port 1, but it only advertises

130.24.0.0 on Port 2. 

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