TCP/IP Connections Introduction User Modes Connect Via... Addressing Subnet Masks Config Router Address DNS Search Paths Hosts File 802.3 Saving Configs Troubleshooting Tools For More Info Glossary updated: 08/11/97

More on Subnet Masks... How Subnet Masks are used to send packets Devices on the same physical network typically have the same network number and same subnet mask. Routers are used to connect individual networks, and they route packets from one network to another as needed. When a node on the network needs to send a packet, it will determine the network number portion of the IP address of a packet, based on IP address and subnet mask. If the destination machine is on the same network as itself, the sender will ARP for the hardware address and sends the packet. If it is not on the same address, it sends the packet to its router. The router also looks at network portion of the address, and checks its routing tables to determine the best route for the packet. Sub-subnetting In many cases, an organization wants to change their subnet masks from the default, to better serve their needs. Example: a university has network address 128.253.0.0. Because the first octet is "128", we know that its a Class B address, with default subnet mask of 255.255.0.0, which means the first 2 octets represent the network address. So this univeristy has 2^16 bits for node addressing, but not all their nodes are on the same network; in fact, they currently have 50 different networks, each with approximately 100 nodes. They could apply a new subnet mask of 255.255.255.0; giving them the third octet to use to define ~256 subnets, each with ~256 node addresses (actually 254 node addresses since node addresses of all zeroes or all one's are reserved for other purposes). In the example below, we see device 1 on subnet 194 on network 128.253. To the outside world (i.e., nodes and routers not at the university itself), the subnet mask for the university address is still 255.255.0.0 (because they recognize the address as a class B address). But once the packet gets delivered to their site, their own routers have been configured to apply the 255.255.255.0 subnet mask to the address, and the university's routers will then determine which of their subnets the packet goes to, and deliver it accordingly. [Intro |UserModes |ConnectVia |Addressing |Subnet Masks |Config|Routers ]DNS |Search Paths |Hosts File ]802.3 ]Saving Configs ]Troubleshooting ]Tools ]More Info ]Glossary ]