Specifications

Niagara Release 2.3
Revised: May 22, 2002 Niagara Networking & Connectivity Guide
Chapter 1 Understanding Networking and IP Addressing
Networking using IP
1–20
For example, in the address 192.168.1.57, the first 3 octets (192.168.1) are the
network portion and the last number (57) is the host number. However, this 3-octet
boundary is not true for all IP addresses. The boundary changes depending upon
which class the address falls into. Classes were designed to accommodate very large
to very small networks, as illustrated in Table 1-6.
Network
(Subnet) Masks
There is a wide difference between the number of unique hosts on a Class B network
(65,534) and a class C network (254). What if you wanted to implement 3 networks
with 1000 hosts each? It would be wasteful to use a class B network for so few hosts,
but a class C is inadequate. Subnetting was developed as a mechanism to break a
single larger network into smaller pieces.
As shown in Table 1-6, each class has a default network mask (sometimes referred
to as the subnet mask). The network mask is used to define the network portion of the
address and indicate whether the network is subnetted. For example, 255.255.255.0
is the network mask for a class C network that has not been subnetted. The mask
255.255.192.0 is an example of a valid mask for a subnetted class B network.
Note Each host on a TCP/IP network requires a subnet mask even on a single-segment
network.
In order to implement a subnet, some bits are borrowed from the host portion of the
address to become the subnet number. In the example above, the subnet number 192
indicates that 2 bits (128+64) were stolen from the 3rd octet. Since those bits are
dedicated to the network function, only 6 bits can be used for host numbers
(providing host numbers 1-63). Consequently, fewer hosts can be defined on any
subnet. Additionally, subnetting uses IP addresses less efficiently because some
addresses are lost to special functions such as the network and broadcast addresses
(see “Special IP Addresses”).
Table 1-6 Internet addressing class system.
Class Network
Prefix
Network to
Host
Boundary
Example
Network
Number
Network Mask Number of
Networks
Number of
Hosts on
Each Network
A 1-126 N.H.H.H 124.0.0.0 255.0.0.0 126 16,777,216
B 128-191 N.N.H.H 145.10.0.0 255.255.0.0 16,384 65,534
C 192-223 N.N.N.H 192.168.1.0 255.255.255.0 2,097,152 254
D 224-239 Reserved for IP multicasting (a form of broadcasting). See “Special IP
Addresses”.
E 240-247 Reserved for experimental use. See “Special IP Addresses”.