BROCADE IP PRIMER

The SubnetTable 2. Understanding SubnetsNetworkAddressSubnetMaskFirst UsableAddressLast UsableAddress172.16.0.0 255.255.254.0 172.16.0.1 172.16.1.255Broadcast AddressNumber of Usable Hosts172.16.1.255 510Why would we want to shrink the host size of a network? Why not just useclassful networking? There's another reason. Your host size determines yourbroadcast domain. Remember how we talked about a broadcast address? Thisis an address that you send a message to that every device in the subnet willreceive.How Does the Network Layer Know Where to Go?We talked a little bit about ARP (Address Resolution Protocol). ARP is a greatexample of a broadcasting protocol. Let's say, we're using a Class A network:10.0.0.0. Because we're using classful networking, its subnet mask would be255.0.0.0. This means eight bits are for the network portion, and 24 bits arefor the host portion. Let's say we have 4,000 devices in this network. Whenone device wants to talk to the other, the Network Layer on the sourcemachine will first check to see if the destination device is in the same network.If it is not, the Network Layer will check the source machine's routing table.A routing table exists on any Layer 3-capable device. This includes workstations.You don't believe your PC has a routing table? Pull up a commandprompt and type: netstat -rn.$ netstat -rnKernel IP routing tableDestination192.168.222.0127.0.0.00.0.0.0Gateway0.0.0.00.0.0.0192.168.222.1Genmask255.255.255.0255.0.0.00.0.0.0FlagsUUUGMSS000Window000irtt000Ifaceeth0loeth0This command will work on most modern operating systems. If not, your operatingsystem will have a way for you to view your machine's routing table. Thetable contains network addresses in one column, and how to get there in theothers. It includes the address of a gateway, as well as which interface theframe should be created on, to get to its destination. A gateway is a device thatis either connected to both networks directly, or it’s a device that knows how toget to a remote network that it's not connected to.Brocade IP Primer 47