Technical data
90362-01 IP Addressing C-3
As figure B-1 shows, the A, B, and C addresses all include a section for the
network ID. This section is called the
subnet
. If the NetBlazer is intercon-
nected to an existing network, assign each LAN interface an unassigned IP
address that uses the same network ID as other hosts on the LAN. Use the
same IP address assigned to one of the LAN interfaces for the NetBlazer’s
global IP address.
If you are setting up a new network that will be attached to the Internet, you
must obtain IP network addresses from the Network Information Center
(NIC). The NIC only assigns the network portion of the address and
delegates responsibility for assigning host addresses to the requesting
organization. For more information on connecting to the Internet, see
Appendix H, “Joining the Global Internet.”
Setting the Subnet Mask
The subnet bitmask tells the NetBlazer how many high-order bits are
assigned to the network ID part of the IP address. You will need to set the
subnet mask based on the class of the IP address you enter. Class B
addresses are usually assigned a 16-bit subnet mask while Class C addresses
are usually assigned a 24-bit subnet mask.
Class A and Class B addresses have a surplus of Host ID bits. Turning some
of those excess Host ID bits into a Subnet ID can:
•
help reduce the number of routing table entries
•
allow decentralized management of host addressing
•
help localize traffic to a subnetwork
The subnet bits are specified by a bit mask called the
subnet mask
. The
subnet mask can have up to 32 bits and is typically represented like an IP
address such as 255.255.255.0. The NetBlazer uses a different style of
representation to simplify things, so the above bitmask is represented as /24
following the network address. The most common subnet bitmask is /24.
As illustrated in Figure B-2, many sites with Class B networks use a 24-bit
subnet mask that allows them to use the third octet of the address to identify
specific physical LANs. The fourth octet identifies a host on the LAN.