计算机网络第5章：OSI层次：传输层.ppt

* * * * * * * * * * * * * * * * * * * * Because many private networks exist alongside public nets, grabbing just any address is strongly discouraged. RFC 1918 sets aside three blocks of IP addresses (i.e., a Class A, a Class B, and a Class C range) for private, internal use. Addresses in this range will not be routed on the Internet backbone . Internet routers immediately discard private addresses.When using private addresses on a network that is connected to the Internet, you should filter packets and routing updates to avoid leaking any RFC 1918 addresses between autonomous systems. For example, if both you and your provider use addresses from the /16 block, your routers could get confused if confronted with updates from both systems. NAT is the process of swapping one address for another in the IP packet header. In practice, NAT is used to allow hosts that are privately addressed (using RFC 1918 addresses) to access the Internet. When a host inside the private domain wants to transmit to a host on the outside world, it forwards the packet to the NAT-enabled device. The NAT process then looks inside the IP header and, if appropriate, replaces the inside IP address with a globally unique IP address. When an outside host sends a response, the NAT process receives it, checks the current table of network address translations, and replaces the destination address with the original inside source. NAT translations can occur dynamically or statically and can be used for a variety of purposes.The most powerful feature of NAT routers is their capability to use port address translation (PAT), which allows multiple inside addresses to map to the same global address. This is sometimes called a many-to-one NAT. This procedure allows for many user inside of the private network to share one single external IP address. When using PAT the NAT router keeps track of the different conversations by mapping TCP and UDP port numbers. * * * * * * access-list 2 permit 55 * * ac