3 A D DI T IOnA L I P-R e L AT eD S e RV IC e S in .NET

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6. 3 A D DI T IOnA L I P-R e L AT eD S e RV IC e S
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Figure 6.11 Host 1 issues a broadcast ARP request asking for Host 2 s MAC address. Host 2 sees the request and sends a unicast ARP reply back to Host 1 with its MAC address.
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192.168.10.1 192.168.10.2 192.168.10.3
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Host 1
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Host 3 Who has 192.168.10.4
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Host 4
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192.168.10.4 is 00:00:11:22:33:AB
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Figure 6.12 In order to reduce the number of ARP requests, IP hosts maintain an ARP cache that contains the answer to previous ARP requests. The entries in the ARP cache are saved for a limited amount of time and then discarded.
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192.168.10.1 192.168.10.2 192.168.10.3
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192.168.10.4 192.168.10.3
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Likewise, the system that responded to the ARP broadcast stores the hardware and IP addresses of the system that sent the original broadcast. If it did not do so, it would have to issue its own ARP broadcast for Host 1 s MAC address to find out where to send the ICMP echo response, which would create unnecessary network traffic. note that IP addresses assigned to a host may not be static and can move around from host
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to host (recall our earlier discussion of DHC). Therefore, entries are removed from the ARP cache after a period of time. If the ARP cache does not time out, then the source may not be able to send its traffic to the right destination host. Several strategies exist that can alleviate the situation, but they are beyond the scope of this book. The previous discussion addressed the use of ARP only within the confines of the same subnet. What happens if the destination host is not in the same subnet, as shown in Figure 6.13 Here, the process is different.
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Figure 6.13 Host 1 wants to send an IP packet to Host 7, but Host 7 is not on its local network. Host 1 determines, based on its IP subnet mask, that Host 7 is on a distant network and issues an ARP request for its default gateway. The router responds with its MAC address, and then Host 1 sends the ICMP echo request to the router for forwarding.
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192.168.10.1 192.168.10.2 172.16.20.1 172.16.20.2
Host 1
Host 3 1 3 4
Host 6 Who has 172.16.20.2
Host 7
Who has 192.168.10.99
192.168.10.99 is 00:00:11:22:33:99 2 192.168.10.99 00:00:11:22:33:99 192.168.10.2 00:00:11:22:33:DD 192.168.10.99 00:00:11:22:33:99 Echo Request to 172.16.20.2 7
172.16.20.2 is 00:00:66:77:88:AB 172.16.20.99 00:00:66:77:88:99 172.16.20.2 172.16.20.1
00:00:66:77:88:AB 00:00:66:77:88:EE
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Host 1 wants to send traffic to Host 7, which is in a remote broadcast domain, and it wants to know if Host 7 can be reached. It tries to ping Host 7. However, in the absence of an ARP entry for 172.16.20.2, it needs to issue an ARP request. But, since 172.16.20.2 is not in the local broadcast domain, it issues an ARP request for its default gateway, which is the router interface in Figure 6.13. Host 1 and Host 3 are programmed with a default gateway address in case they need to connect to hosts outside their local domain. For a local host to contact a remote host, it must send an ARP request to the default gateway. The router receives the broadcast on its interface in the 192.168.10.0 domain and issues an ARP response with its MAC address, and Host 1 can now form the IP packet for Host 7. Host 1 creates an IP packet that is then sent in an ethernet frame to the router.
6. 3 A D DI T IOnA L I P-R e L AT eD S e RV IC e S
Using its forwarding table, the router decides to forward the packet out of the second interface. However, it does not have an ARP entry for the host 172.16.20.2. It therefore issues an ARP request in this broadcast domain. Upon receiving the ARP request, Host 7 responds with a unicast ARP response to the router. note that ARP only works within the scope of a broadcast domain and therefore is never forwarded by a router. Just as with an IP host, the router maintains an ARP cache listing entries for the different broadcast domains it is connected to. Listing 6.2 shows a packet capture of an ARP request. The capture shows a host with IP address 138.120.53.253 that is attempting to resolve the MAC address for a host with IP address 138.120.53.149. The destination MAC address of the ethernet II frame is sent to the broadcast address ff:ff:ff:ff:ff:ff. All devices in the same broadcast domain will receive this frame. Only the host with IP address 138.120.53.149 will reply. The etherType for ARP is 0x0806 and is used to indicate which protocol is being transported in the ethernet II frame.