 (without any MAC protocol) to router 2 at the other edge of the network. Router 2 knows from stored routing information that the destination IP address is located in the direction of router 3 and forwards the IP packet to that direction through network 2, which may use frame-relay technology below the network layer. Router 3 receives the IP packet, detects that it belongs to host B whose MAC address it knows, and attaches it to Ethernet frame together with the MAC addresses for itself and host B. The goal of this example is to make clear the relationship between MAC and IP addresses and to illustrate data flow through protocol stacks, which was discussed in generic terms in Section 6.3. In later sections we describe these TCP/IP processes in detail. 6.6.3 Bearer Network Protocols for IP Most of the bearer networks mentioned in Figure 6.36 were discussed in Sections 6.4 and 6.5. Both physical and data link layer protocol functions are needed to transmit IP packets from a host to a router and from a router to another router, as shown in Figure 6.37. The Ethernet frame carries hard- ware or MAC addresses that make up a point-to-point connection over shared media between two machines as explained in Section 6.5. The Ether- net frame also contains error check and network layer protocol information as explained in Section 6.5.4. Other bearer alternatives in Figure 6.36, such as PSTN and ISDN, carry data point to point and perform physical layer tasks. They do not con- tain data link layer functionality, and a protocol, such as SLIP or PPP, is needed to frame IP packets for physical transmission over serial lines. SLIP is a simple framing protocol used to send IP packets across a tele- phone line. The problems with SLIP are that many incompatible versions of SLIP are in use, it does not do any error control, and it is not able to assign IP addresses dynamically. PPP is a more modern protocol, which solves all the problems of SLIP and can also send other protocols in addition to IP. PPP is used in many applications from