CHAPTER 1: INTRODUCTION1-26EMM-E6 User’s GuideSubnet addresses, when used with routing, allow discrimination betweendevices and groups of devices based on IP addresses. Networks ofdifferent subnets, even those on the same physical network segment, maybe isolated, from a functional standpoint, from one another through theimplementation of routing. Repeaters, bridges, and switches, whichoperate at the Data Link layer of the OSI model, make their decisionsbased on MAC addresses. Network devices such as routers, servers, andclient stations can use IP addressing to recognize transmissions intendedfor them. If a station on one routed subnet sees a transmission fromanother subnet, it will ignore the packet without concern over who it isintended for. To overcome this subnetwork blindness a router is used. Anystation or device which implements subnet masking needs to beconfigured with an address for that subnet’s Default Gateway. When thestation or device transmits packets intended for a different subnetworkthan the one it identifies itself as belonging to, the transmission is alsosent to the Default Gateway, where the gateway or router will make thedetermination of where the packet is sent.The use of subnet addresses on the network means using a Subnet Maskin conjunction with each IP address.1.10.5 Subnet MasksThe purpose of the Subnet Mask is to indicate the part of the Host ID thatis being used as a subnet address. By default no part of the Host ID isused, and therefore, the default or “Natural Mask” masks just the octetsthat comprise the Network ID. Table 1-5 shows the default masks for thefour classes of IP networks.Table 1-5. Class and Default MasksNetwork Class Length of Network ID Default MaskClass A X. 255. 0. 0. 0Class B X. X. 255. 255. 0. 0Class C X. X. X. 255. 255. 255. 0
