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Unformatted text preview: t show a linklayer header, as we do on the Ethernets, because there isn't one on a SLIP link.
When netb receives the datagram it goes through the same steps that sun just did: the datagram is
not destined for one of its own IP addresses, and netb is configured to act as a router, so the
datagram is forwarded. The default routing table entry is used, sending the datagram to the nexthop router gateway (126.96.36.199). ARP is used by netb on the Ethernet 140.252.1 to obtain the
48-bit Ethernet address corresponding to 188.8.131.52, and that Ethernet address is the destination
address in the link-layer header.
gateway goes through the same steps as the previous two routers and its default routing table
entry specifies 184.108.40.206 as the next-hop router. (We'll verify that this is the next-hop router
for gateway using Traceroute in Figure 8.4.) file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/ip_inter.htm (8 of 19) [12/09/2001 14.46.37] Chapter 3. IP: Internet Protocol Figure 3.4 Initial path of datagram from bsdi to ftp.uu.net (220.127.116.11).
A few key points come out in this example.
1. All the hosts and routers in this example used a default route. Indeed, most hosts and some
routers can use a default route for everything other than destinations on local networks.
2. The destination IP address in the datagram never changes. (In Section 8.5 we'll see that this
is not true only if source routing is used, which is rare.) All the routing decisions are based
on this destination address.
3. A different link-layer header can be used on each link, and the link-layer destination address
(if present) always contains the link-layer address of the next hop. In our example both
Ethernets encapsulated a link-layer header containing the next-hop's Ethernet address, but
the SLIP link did not. The Ethernet addresses are normally obtained using ARP.
In Chapter 9 we'll look at IP routing again, after describing ICMP. We'll also look at some sample
routing tables and how they're used...
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