Lucent Technologies Release 8.2 manual Class Type Network ID Host ID, Default Subnet Mask

IP Addressing 1 Networking Overview

Two-level classful hierarchy

Class

Type

Network ID

Host ID

Three-level subnet hierarchy

Class

Type

Network ID

Subnet ID

Host ID

Extended Network Prefix

Subnet mask

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

	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.”
			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
	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.
Administration for Network Connectivity
555-233-504 — Issue 1 — April 2000			CID: 77730	17