TCP IP Illustrated

The router is acting as a proxy agent for the

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Unformatted text preview: P. In Figure 3.10 we showed that the system sun was connected to two Ethernets. But we also noted that this wasn't really true, if you compare that figure with the one on the inside front cover. There is in fact a router between sun and the subnet 140.252.1, and this router performs proxy file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/arp_addr.htm (8 of 12) [12/09/2001 14.46.39] Chapter 4. ARP: Address Resolution Protocol ARP to make it appear as though sun is actually on the subnet 140.252.1. Figure 4.6 shows the arrangement, with a Telebit NetBlazer, named netb, between the subnet and the host sun. Figure 4.6 Example of proxy ARP. When some other host on the subnet 140.252.1 (say, gemini) has an IP datagram to send to sun at address, gemini compares the network ID (140.252) and subnet ID (1) and since they are equal, issues an ARP request on the top Ethernet in Figure 4.6 for IP address The router netb recognizes this IP address as one belonging to one of its dialup hosts, and responds with the hardware address of its Ethernet interface on the cable 140.252.1. The host gemini sends the IP datagram to netb across the Ethernet, and netb forwards the datagram to sun across the dialup SLIP link. This makes it transparent to all the hosts on the 140.252.1 subnet that host sun is really configured "behind" the router netb. If we execute the arp command on the host gemini, after communicating with the host sun, we see that both IP addresses on the 140.252.1 subnet, netb and sun, map to the same hardware address. This is often a clue that proxy ARP is being used. gemini % arp -a many lines for other hosts on the 140.252.1 subnet netb ( at 0:80:ad:3:6a:80 sun ( at 0:80:ad:3:6a:80 Another detail in Figure 4.6 that we need to explain is the apparent lack of an IP address at the bottom of the router netb (the SLIP link). That is, why don't both ends of the dialup SLIP link have an IP address, as do both ends of the hardwired SLIP link between bsdi and slip? We noted i...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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