Cabletron Systems 1800 Using the Node as an IP Gateway

14-2 Smart Switch 1800 4. 0 User Guide , Rev 0 1

●Class C allows up to approximately 2 million network numbers, and 255 node

numbers: Bits 0–2 are 110, bits 3–23 are t he network identifier, a nd bits 24–31

are the node identifier. In dotted decimal notation, the range of Class C

addresses i s 192.0.0.0–223.255.255.255 .

If the node identifier is all zeroes, the overall 32-bit IP address refers specifically to

the network identified by the network identifier. If the host identifier is all ones, the

32-bit address refers to all hosts attached to the network. (This is the IP broadcast

address for that network.) If the entire 32 bits are ones, the address refers to all hosts

on the local network.

IP addresses are administered by Standford Research Institute’s Network

Information Center (NIC).

Multiple physical networks can use a single IP network address. This is done by using

a portion of the node identifier (it does not need to be in co mplete octets) as a "subnet"

identifier. For example, in a Class B network, the first and second octets are the

network identifier, and the third and fourth are the node identifier. If the network con-

tains the maximum of 65,025 nodes, management w ill most likely be easier if the third

octet of the IP address is used to define up to 255 subnets on the network. The fourth

octet can then define up to 255 nodes on each subnet, effectively providing unique

identifiers for all 65,025 nodes on the network.

As described earlier, the first octet of the IP address identifies the network’s class:

0–127 = A, 128–191 = B, and

192–223 = C. Once the Internet Protocol knows the class ,

it knows how many octets are supposed to identify the network and how many are s up-

posed to identify th e node. When part of th e node identifier is used as a subnet iden-

tifier, there must be a method of telling IP of the modification. This is done with a

subnet mask. The mask is created by taking the 32-bit (binary) IP address, then sub-

stituting a 1 for every bit that identifies the network or a subnet, and a 0 for every bit

that identifies a node. When IP applies this mask to an actual IP address, it can see

which bits identify the subnet and which identify the node. (IP already knows the

network’s class, and therefore which bits identify the network.)

Using the Node as an IP Gateway

A Smart Switc h 18 00 can functio n as an I P gatew ay, and f orward IP pac kets. Th is

requires that IP be configured (and loaded) in the Protocols file (described in

Chapter 5).

Network Node