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24-port NWay Ethernet Switch User’s Guide
122 Switch Management and Operating Concepts
that the remaining octets must be zero. The field labeled
FAMILY OF NET 1 identifies the protocol family under which
the network address should be interpreted. RIP uses values
assigned to address families under the 4BSD UNIX operating
system (IP addresses are assigned a value of 2).
In addition to normal IP addresses, RIP uses the convention that
address 0.0.0.0 denotes a default route. RIP attaches a distance
metric to every route it advertises, including default routes.
Thus, it is possible to arrange for two routers to advertise a
default route (for example, a route to the Internet) at different
metrics, making one of them a primary path and the other a
backup.
The final field of each entry in a RIP message, DISTANCE TO
NET 2, contains an integer count of the distance to the specified
network. Distances are measured in router hops, but values are
limited to the range 1 through 16, with the distance 16 used to
signify infinity (unreachable).
RIP 1 Route Interpretation and
Aggregation
Because RIP was originally designed to be used with classful
addresses, version 1 did not include any provision for a subnet
mask. When subnet addressing was added to IP, version 1 of RIP
was extended to permit routers to exchange subnetted addresses.
However, because RIP 1 update messages do not contain explicit
mask information, an important restriction was added – a router
can include host-specific or subnet-specific address in routing
updates as long as all receivers can unambiguously interpret the
addresses. In particular, subnet routes can be included in
updates sent across a network that is part of the subnetted
prefix, and only if the subnet mask used with the network is the
same as the subnet mask used with the address. The restriction