tailieunhanh - Internetworking with TCP/IP- P19

Internetworking with TCP/IP- P19: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 148 Classless And Subnet Address Extensions CIDR Chap. 10 The chief advantage of dividing an IP address into two parts arises from the size of the routing tables required in routers. Instead of keeping one routing entry per destination host a router can keep one routing entry per network and examine only the network portion of a destination address when making routing decisions. Recall that the original IP addressing scheme accommodated diverse network sizes by dividing host addresses into three primary classes. Networks assigned class A addresses partition the 32 bits into an 8-bit network portion and a 24-bit host portion. Class B addresses partition the 32 bits into 16-bit network and host portions while class C partitions the address into a 24-bit network portion and an 8-bit host portion. To understand some of the address extensions in this chapter it will be important to realize that individual sites have the freedom to modify addresses and routes as long as the modifications remain invisible to other sites. That is a site can choose to assign and use IP addresses in unusual ways internally as long as All hosts and routers at the site agree to honor the site s addressing scheme. Other sites on the Internet can treat addresses as a network prefix and a host suffix. Minimizing Network Numbers The original classful IP addressing scheme seems to handle all possibilities but it has a minor weakness. How did the weakness arise What did the designers fail to envision The answer is simple growth. Because they worked in a world of expensive mainframe computers the designers envisioned an internet with hundreds of networks and thousands of hosts. They did not foresee tens of thousands of small networks of personal computers that would suddenly appear in the decade after TCP IP was designed. Growth has been most apparent in the connected Internet where the size has been doubling every nine to fifteen months. The large population of networks with trivial size .