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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