tailieunhanh - Internetworking with TCP/IP- P20

Internetworking with TCP/IP- P20: 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 | 158 Classless And Subnet Address Extensions CIDR Chap. 10 Routing In The Presence Of Subnets The standard IP routing algorithm must be modified to work with subnet addresses. All hosts and routers attached to a network that uses subnet addressing must use the modified algorithm which is called subnet routing. What may not be obvious is that unless restrictions are added to the use of subnetting other hosts and routers at the site may also need to use subnet routing. To see how a problem arises without restrictions consider the example set of networks shown in Figure . In the figure physical networks 2 and 3 have been illegally assigned subnet addresses of a single IP network address N. Although host H does not directly attach to a network that has a subnet address it must use subnet routing to decide whether to send datagrams destined for network N to router RI or router R2. It could be argued that H can send to either router and let them handle the problem but that solution means not all traffic will follow a shortest path. In larger examples the difference between an optimal and nonoptimal path can be significant. Net 1 not a subnet address Net 2 subnet of address N R2 Net 3 subnet of address N Figure An example illegal topology with three networks where Nets 2 and 3 are subnets of a single IP network address N. If such topologies were allowed host H would need to use subnet routing even though Net 1 does not have a subnet address. In theory a simple rule determines when machines need to use subnet routing. The subnet rule is To achieve optimal routing a machine M must use subnet routing for an IP network address N unless there is a single path P such that P is a shortest path between M and every physical network that is a subnet of N. Unfortunately understanding the theoretical restriction does not help in assigning subnets. First shortest paths can change if hardware fails or if routing algorithms redirect traffic around congestion. Such dynamic