7-2 CHAPTER 7: ADMINISTERING IP ROUTING
packets to or from the switching module itself. Second, the IP address
defines the network and subnet numbers of the segments connected to
that interface.
Packets to be forwarded by the switching module contain the IP addresses of
the original source and the ultimate destination.
■Subnet Mask
A subnet mask is a 32 bit number that uses the same format and
representation as IP addresses. The subnet mask determines which bits in
the IP address are interpreted as the network number, the subnet number,
and the host number. Each IP address bit corresponding to a 1 in the
subnet mask is in the network/subnet part of the address. Each IP address
bit corresponding to a 0 is in the host part of the IP address.
■Broadcast Address
This is the IP address to be used by the switching module when it
broadcasts packets to other stations on the same subnet. In particular, this
address is used for sending RIP updates. By default the switching module
uses a directed broadcast (all ones in the host field).
■Cost
This is a number between one and fifteen that is used when calculating
route metrics. Unless your network has special requirements, you should
assign a cost of 1 to all interfaces.
■Ports
A single interface may contain several bridge ports. All of the ports
corresponding to one interface share the same IP address, subnet mask,
broadcast address, and cost. An ESM contains nine ports: one FDDI and nine
Ethernet. The port indices are always the following: 1 = FDDI and 2 – 9 =
Ethernet. An EFSM contains a maximum of eighteen ports: two FDDI and
sixteen Ethernet. The port indices for the maximum configuration are the
following: 1, 2 = FDDI and 3 – 18 = Ethernet.
You do not have to include every switching module port in an interface
(that is, some ports may remain unassigned). Packets will be bridged to and
from unassigned ports. However, IP packets will not be forwarded to ports
that are not assigned to any IP interface.