TCP/IP是如何工作的（一）

　　Over the past few years, one of the most written－about network topics has been IP. But even with all this attention, few, if any, stories have traced the protocol\'s basic workings, that is, how routers and Layer 3 switches act upon IP information to move Ethernet packets across the network.

　　As a point of referenced, bear in mind that IP is a member of the TCP/IP protocol suite.

　　TCP functions at the Open Systems Interconnection（OSI）transport layer, or Layer 4.Its chief responsibility is to ensure reliable end－to－end connectivity. IP, located one layer down, at the OSI network layer, or Layer 3, communicates the addresses of each packet\'s sender and receiver to the routers along the way. Routers and Layer 3 switches can read IP and other Layer 3 protocols .This information, combined with routing tables and other network intelligence, is all it takes to get across the room or around the world via TCP/IP.

　　The routing process begins with an IP address that is unique to the sending end station .End stations may be assigned permanent IP addresses or they may borrow them as needed from a Dynamic Host Configuration Protocol（DHCP）server or other service.

　　Each packet carries a source address, which under current（IPv4）specifications is 32 bits long .In its header, each packet also carries the IP address of the final destination.

　　If the sending end station determines that the destination address is not local, the packet goes to a first－hop router, typically one that is close and has been preassigned to the sender.

　　The router inspects the packet\'s IP address and performs a route table lookup to see if the destination end station resides on the local（physically connected）network, typically called an IP subnet .An IP subnet usually is assigned to each of the router\'s network interfaces.