address field by simply counting down from the unique start-of-frame octet. It knows a priori because the router processor conforms to a particular protocol. 2. What is the addressing capacity of an address field? We will let each address consists of a distinct 8- or 16-bit binary sequence. For an 8-bit binary group, how many distinct 12.1 WIDE AREA NETWORK DEPLOYMENT 8-bit sequences are there? Remember, on developing code capacity (Section 10.4), that we can have 2nd sequences where n is the number of bits in a particular sequence. In our sample case it is 8 bits. Thus, its addressing capacity is 28 or 256 distinct addresses. The addressing capacity can be increased by using extended addressing, namely where we use two octets for a destination address. Often the least significant bit of the first octet is reserved to tell the receiver processor to expect the next octet also to be dedicated to destination address. Now we have only 7 bits in the first octet left for addressing and all 8 bits in the second octet. Adding these two together, we get 15 bits for addressing. Thus, there is 2 15 or 32,768 distinct addresses, quite a respectable number. 3. How does a router or data switch know how to route to a particular address? Simply by consulting a look-up table. Now this look-up table may have fixed routing entries, or entries that can be updated manually or routing entries that are updated dynamically. Here we must recognize three possible conditions: 1. A node/router is added or dropped from the network. 2. New routing patterns may be established; other routing patterns may be discontinued. 3. There may be congestion, route/node degradation, and/or failure. Read subsection 12.3 which describes the TCP/IP protocol family because IP has some very interesting means and methods to update routing/look-up tables as well as finding routes to "unknown" destination addresses. Error detection and correction were discussed in Section 10.5. 12.1.1.2 There May Be an Issue of Urgency. There are several "families" of data messag