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IP Addressing
17
Administration for Network Connectivity
555-233-504 — Issue 1 — April 2000 CID: 77730
1 Networking Overview
The extended network prefix is then treated as a normal network ID. The remaining
host ID bits define the host IDs within each subnet. For example, a block of IP
addresses could be subdivided into four subnets by using 2 host bits to “extend” the
network ID. Now there are 4 times as many (extended) networks and 1/4 as many
hosts per network.
Note: In adding up the number of network and host IDs, certain addresses
cannot be counted. In general, addresses with all ones or all zeros in
either the network portion or the host portion of the address are not
usable. These are reserved for special uses, such as broadcasting or
loopback.
Subnet Masks Routing protocols use a subnet mask to determine the boundary between the extended
network ID and the host ID in an IP address. The subnet mask is a 32-bit binary
number consisting of a string of contiguous 1’s followed by a string of contiguous
0’s. The 1’s part corresponds to the extended network prefix and the 0’s part
corresponds to the host ID of the address.
Each of the three classes of addresses has a default subnet mask that specifies the end
of the 1st, 2nd, and 3rd octet as the boundary between the extended network prefix
and the host ID. The default subnet mask in each case means “no subnetting.”
In addition to the default subnet masks, which divide the network and host IDs at the
octet boundaries in the IP address, subnets can be formed by using 2 or more bits
from the host octets to define the subnet ID.
Two-level classful hierarchy
Three-level subnet hierarchy
Subnet mask
Class
Type
Network ID Host ID
Class
Type
Network ID Subnet ID Host ID
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 . . . 1 0 0 0 0 0 . . . 0
Extended Network Prefix
Default Subnet Mask
Class A 11111111.00000000.00000000.00000000
255.0.0.0
Class B 11111111.11111111.00000000.00000000
255.255.0.0
Class C 11111111.11111111.11111111.00000000
255.255.255.0