Data Communications Networking Devices - 4th Ed.pdf

568 ________________________________________________ LAN INTERNETWORKING DEVICEShas a distinct network address and each interface on the network has a unique hostaddress that represents the host portion of a 32-bit address. Since the IP addressoccurs at the network layer while frames that move data on a LAN use MACaddresses associated with the data link layer, a translation process is required toenable IP-compatible devices to use the transport services of a local area network.Thus, any discussion of how routers support IP requires an overview of the mannerby which hosts use the services of a router.When a host has a packet to transmit, it will ®rst determine if the destination IPaddress is on the local network or a distant network, with the latter requiring theservices of a router. To accomplish this, the host will use the subnet mask bits set inits con®guration to determine if the destination is on the local network. Forexample, assume the subnet mask is 255.255.255.128. This means the mask extendsthe network portion of an IP address to 11111111.11111111.11111111.1, or 25 bitpositions, resulting in seven 32 25) bit positions being available for the hostaddress. This also means you can have two subnets, with subnet 0 containing hostaddresses 0 to 127 and subnet 1 having host addresses 128 to 255, the subnet beingde®ned by the value of the 25th bit position in the IP address.If we assume the base network IP address is 193.56.45.0, then the base network,the two subnets, and the subnet mask are as follows:Base network: 11000001.00111000.00101101.00000000 = 193.56.45.0Subnet 0: 11000001.00111000.00101101.00000000 = 193.56.45.0Subnet 1: 11000001.00111000.00101101.10000000 = 193.56.45.128Subnet mask: 11111111.11111111.11111111.10000000 = 193.56.45.128Now suppose a host with the IP address 193.56.45.21 needs to send a packet tothe host whose address is 193.56.45.131. By using the subnet mask, the transmittinghost notes that the destination, while on the same network, is on a differentsubnet. Thus, the transmitting host will require the use of a router in the samemanner as if the destination host was on a completely separate network.Figure 5.17 illustrates the internal and external network view of the subnettednetwork. Note that from locations exterior to the network, routers forward packetsFigure 5.17Using subnet masks to subdivide a common IP network address