tailieunhanh - Internetworking with TCP/IP- P67

Internetworking with TCP/IP- P67: 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 | Sec. Interface Identifiers 619 0 8 24 47 b Figure a The format of a 48-bit IEEE 802 address used with Ethernet with bits labeled c specifying the company that manufactured the interface and bits in the man. ext. field specifying an extension the manufacturer chose to uniquely identify the unit and b the encoding of the address in the low order 64 bits of an IPv6 unicast address. Additional Hierarchy Although the unicast address format in Figure implies a strict hierarchy many additional levels are possible. For example bits of the NLA ID can be used to create a hierarchy of providers. Similarly the 16-bit SLA ID can be divided to create a hierarchy within an organization. The large number of bits provides more flexibility than IPv4 subnetting. An organization can choose to divide into a two-level hierarchy of areas and assign subnets within each area. Alternatively an organization can choose a three-level hierarchy of areas subareas and subnets within each subarea. Local Addresses In addition to the global unicast addresses described above IPv6 includes prefixes for unicast addresses that have local scope. As Figure shows the standard defines two types a link-local address is restricted to a single network and a site-local address is restricted to a single site. Routers honor the scoping rules they do not forward datagrams containing locally-scoped addresses outside the specified scope. Local addresses solve two problems. Link-local addresses provide communication across a single physical network without danger of the datagram being forwarded across the internet. For example when it performs neighbor discovery an IPv6 node uses a link-local address. The scope rules specify that only computers on the same physical network as the sender will receive neighbor discovery messages. Similarly computers connected to an isolated network . a network that does not have routers attached can use link-local addresses to communicate. 620 The