EMM-E6 Ethernet User's Guide

CHAPTER 1:

INTRODUCTION

1-26 EMM-E6 User’s Guide

Subnet addresses, when used with routing, allow discrimination between

devices and groups of devices based on IP addresses. Networks of

different subnets, even those on the same physical network segment, may

be isolated, from a functional standpoint, from one another through the

implementation of routing. Repeaters, bridges, and switches, which

operate at the Data Link layer of the OSI model, make their decisions

based on MAC addresses. Network devices such as routers, servers, and

client stations can use IP addressing to recognize transmissions intended

for them. If a station on one routed subnet sees a transmission from

another subnet, it will ignore the packet without concern over who it is

intended for. To overcome this subnetwork blindness a router is used. Any

station or device which implements subnet masking needs to be

conﬁgured with an address for that subnet’s Default Gateway. When the

station or device transmits packets intended for a different subnetwork

than the one it identiﬁes itself as belonging to, the transmission is also

sent to the Default Gateway, where the gateway or router will make the

determination of where the packet is sent.

The use of subnet addresses on the network means using a Subnet Mask

in conjunction with each IP address.

1.10.5 Subnet Masks

The purpose of the Subnet Mask is to indicate the part of the Host ID that

is being used as a subnet address. By default no part of the Host ID is

used, and therefore, the default or “Natural Mask” masks just the octets

that comprise the Network ID. Table 1-5 shows the default masks for the

four classes of IP networks.

Table 1-5. Class and Default Masks

Network Class Length of Network ID Default Mask

Class A X. 255. 0. 0. 0

Class B X. X. 255. 255. 0. 0

Class C X. X. X. 255. 255. 255. 0

CH1Book Page 26 Wednesday, March 20, 1996 7:48 AM