Operation Manual

AT-RG213 Residential VoIP Gateway - SIP Software Reference Manual
SIP Software Release 6-0-0
J613-M0524-00
22
Figure 5. Subdivision of the 32 bits of an Internet address into network
and host fields for class A, B and C networks
1 7 24
0
NETWORK
HOST
CLASS A
1 1 14 16
0NETWORK
HOSTCLASS B
1 1 1 21 8
0
NETWORK HOSTCLASS C
1
11
The addressing scheme is designed to allow routers to efficiently extract the
host and network portions of an address. In general a router is only
interested in the network portion of an address.
Class A sets the Most Significant Bit (MSB) to 0 and allocates the next 7 bits
to define the network and the remaining 24 bits to define the host. Class B
sets the two MSBs to 10 and allocates the next 14 bits to designate the
network while the remaining 16 refer to the host. Class C sets the three MSBs
to ‘110’ and allocates the next 21 bits to designate the network while the
remaining 8 are left to the user to assign as host or subnet numbers.
The term host refers to any attached device on a subnet, including PCs,
mainframes and routers. Most hosts are connected to only one network. In
other words they have a single IP address. Routers are connected to more
than one network and can have multiple IP addresses. The IP address is
expressed in dotted decimal notation by taking the 32 binary bits and
forming 4 groups of 8 bits, each separated by a dot.
For example:
10.4.8.2 is a class A address
10 is the DDN assigned network number
.4.8 are (possibly) user assigned subnet numbers
.2 is the user assigned host number
172.16.9.190 is a class B address