BROCADE IP PRIMER

BI4XGBI24CT X RX T X RX T X RX T X RXBI4XGBI24CT X RX T X RX T X RX T X RXBI24C6 12 18 2430 36 42 48BI24CBigIron RX-8NETWORKS1 7 13 1925 31 37 43BI24CBI24CBIBI24C4XGT X RX T X RX T X RX T X RXBIBI24C4XGT X RX T X RX T X RX T X RXAC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYSThe SubnetSource10.1.1.201:23:45:67:89:ABDestination192.168.100.202:04:06:08:0A:0CSource10.1.1.202:04:06:08:0A:DEDestination192.168.100.203:02:01:AB:CD:EFHost A10.1.1.2255.255.255.001:23:45:67:89:AB10.1.1.1255.255.255.002:04:06:08:0A:0C192.168.100.1255.255.255.002:04:06:08:0A:DEHost B198.168.100.2255.255.255.003:02:01:AB:CD:EFThis is why the gateway needs to be part of the same network as the host. Thehost needs to know how to get to the gateway. It will communicate to the gatewayusing its MAC address. If it doesn't know the MAC address, it sends anARP broadcast. If the gateway is not in the same network (or broadcastdomain), the gateway will never see the ARP broadcast. Broadcasts neverleave the broadcast domain. If the gateway never sees the ARP broadcast, itwill never reply to it. The gateway must have an address in the same networkas the hosts that use it.CIDRWith classless networking, there came a new way to label a network. Withclassful networking, you could see a network address of, say, 10.0.0.0, andyou would know that this was a Class A network, and therefore, its subnetmask must be 255.0.0.0. With classless networking, you could have a subnetlength of eight bits, 13 bits, 29 bits, almost anything! A new naming schemahad to easily reflect the subnet mask.Enter Classless Inter-Domain Routing (CIDR). It's usually pronounced like“cider.” The convention states that you can represent the number of networkbits in a subnet mask, by adding a number after a slash (“/”) character. Forexample, 10.0.0.0, in classful networking, has a subnet mask of 255.0.0.0(Class A). This means that the network portion has eight bits. Using CIDR, wewould represent this same network like this: 10.0.0.0/8. Notice that I used thenetwork address (10.0.0.0), followed it by a slash (/), and then followed that bythe number of network bits in the subnet mask (8, in this example).Using CIDR notation, you can easily represent any subnet. Let's look back at acouple of our previous examples. Remember the network that needed 400hosts? We came up with a subnet mask of 255.255.254.0, or 23 network bits.We used a Class B network to start with-172.16.0.0. The CIDR notation wouldbe 172.16.0.0/23. Or, how about the network where we only needed twohosts? We started with a Class C network, 192.168.100.0. We decided on asubnet mask of 255.255.255.252, or 30 network bits. The CIDR notationwould then be 192.168.100.0/30.Classless networking is the default today. Get to know CIDR notation well. It isnot only used throughout this book, it's used throughout the networking industry.It's a concise way to indicate the network address and the subnet mask ofa network.Brocade IP Primer 51