with the help of the postal service) to the destination. We, the originator, are completely unaware of the letter's routing; we don't really care. In the data world, this is called connectionless service. In the case of the postal service, the address on the envelope routed the letter. For the data message case, it is the header that carries the routing information. In many situations, a data message may be made up of a number of frames. Each frame carries a field set aside for the recipient address. More often than not it is called the destination address. In many cases, a frame also carries the originator's address. In either case, these are 8-bit fields, often extendable in increments of 8 bits. Three key questions come to mind. (1) What part of a frame's header can be eas- ily identified as the originator and destination address fields? (2) What is the addressing capacity of an address field? Rephrased: How many distinct addresses can be accommo- dated in an 8-bit field? (3) Once a router, smart bridge, or data switch recognizes the boundaries of (a) destination address(es), how does it know how to route the frame? 1. A family of protocols governs the operation of a particular data network. Our interest here is in the network layer and data-link layer protocols. These protocols carefully lay out the data-link frame and the network layer frame (see Section 10.10). For example, in Section 10.7.2 we introduced synchronous transmission and the generic data-link layer frame (Figure 10.7). One thing a digital processor can do and do very well is count bits, and groups of 8 bits, which we call an octet (others call it a byte). A specific network utilizes a particular data-link layer protocol. Thus a processor knows a priori where field boundaries are, because it is designed to meet the requirements of a particular protocol. With some data-link protocols, however, there may be a variable length info field. It is obvious that if this is the case, that the processor must be informed of the length of a particular info f