User`s guide

Configuring IP Routing
Introduction to IP routing and interfaces
MultiVoice Gateway for the MAX— User’s Guide Preliminary November 23, 1998 8-3
Zero subnets
Early implementations of TCP/IP did not allow zero subnets. That is, subnets could have the
same base address that a class A, B, or C network would have. For example, the subnet
192.168.8.0/30 was illegal because it had the same base address as the class C network
192.168.8.0/24, while 192.168.8.4/30 was legal (192.168.8.0/30 is called a zero subnet,
because like a class C base address, its last octet is zero). Modern implementations of TCP/IP
allow subnets to have base addresses that might be identical to the class A, B, or C base
addresses. Ascend’s implementations of RIP 2 and OSPF treat these so-called zero
subnetworks the same as any other network. You should decide whether or not to support and
configure zero subnetworks for your environment. If you configure them in some cases and
treat them as unsupported in other cases, you will encounter routing problems.
Table 8-2 shows how the standard subnet address format relates to Ascend notation for a class
C network number.
The broadcast address of any subnet has the host portion of the IP address set to all ones. The
network address (or base address) represents the network itself, with the host portion of the IP
address set to all zeros. Therefore, these two addresses define the address range of the subnet.
For example, if the MultiVoice Gateway configuration assigns the following address to a
remote router:
IP address = 198.5.248.120
Mask = 255.255.255.248
The Ethernet attached to that router has the following address range:
198.5.248.120 — 198.5.248.127
A host route is a special case IP address with a subnet mask of 32 bits. It has a subnet mask of
255.255.255.255.
Table 8-2. Standard subnet masks
Subnet mask Ascend notation Number of host addresses
255.255.255.0 /24 254 hosts + 1 broadcast, 1 network (base)
255.255.255.128 /25 126 hosts + 1 broadcast, 1 network (base)
255.255.255.192 /26 62 hosts + 1 broadcast, 1 network (base)
255.255.255.224 /27 30 hosts + 1 broadcast, 1 network (base)
255.255.255.240 /28 14 hosts + 1 broadcast, 1 network (base)
255.255.255.248 /29 6 hosts + 1 broadcast, 1 network (base)
255.255.255.252 /30 2 hosts + 1 broadcast, 1 network (base)
255.255.255.254 /31 invalid netmask (no hosts)
255.255.255.255 /32 1 host — a host route