1. The IPU with IP address ipa3 sends an IP-SDU to ipa1. The packet switch PSX consults its RA, which finds the route re1, and the switch relays the IP-IDU over that route. This procedure only requires two columns in the routing table: the destination IP address and the local route identifier. However, if you refer to Fig. 3.73, you see that there are different IP-UDTs handling different routes. There- fore an additional column is needed (called "Transport" in Fig. 3.75) to identify (in a local way) the IP-UDT that handles the route. I have identified the IP-UDTs by their user role in relation to the actual external transport network. The Trans- port column is basically the only thing that separates routing in IP networks from routing in any kind of network. 2. The IPU with IP address ipa3 sends an IP-SDU to ipa6. The packet switch PSX consults its RA, which finds the route re6, and the switch relays the IP-IDU over that route. PSY consults its RA, which finds the route re3 (assumed to be real- ized in an Ethernet), and the switch relays the IP-IDU over that route. The IPU with IP address ipa3 sends an IP-SDU to ipa57, which will be relayed over re6, for the same reason as in the previous scenario. PSY consults its RA, which also has no entry for ipa57, therefore the packet will be relayed to the switch PSZ (assumed to be realized in an ATM network). These scenarios would have looked similar if we had discussed routing in a cir- cuit-switched network. The real difference becomes visible if we analyse how transport of IP-IDUs works over the routes. Figure 3.76 shows the scenario for transport of an IP-SDU over the route PSX/re6-PSY/re1. 3 Logical Network Modelling source dest. route Addr. Addr. IP-SDU IP-IDU IP-PDU userData AAL3/4-PDU AAL3/4- AAL3/4- IP-IDU AAL3/4U IP-IDU Route ATM address Adress (aa3) IP-SDU Translator Fig. 3.76 Transport of an IP IDU over ATM Since IP routes depend on external transport networks, which have no knowl- edge of IP addresses, the IP destination address must first be translated to a 