TCP IP Illustrated

2521363 255255255224 1402521332 31 broadcast addr

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Unformatted text preview: y are multihomed and route packets from one interface to another. The final row in this table notes that the broadcast address for the Ethernet in Figure 3.10 is 140.252.13.63: it is formed from the subnet ID of the Ethernet (140.252.13.32) and the low-order 5 bits in Figure 3.11 set to 1 (16+8+4+2+1 = 31). (We'll see in Chapter 12 that this address is called the subnet-directed broadcast address.) 3.8 ifconfig Command Now that we've described the link layer and the IP layer we can show the command used to configure or query a network interface for use by TCP/IP. The ifconfig(8) command is normally run at bootstrap time to configure each interface on a host. For dialup interfaces that may go up and down (such as SLIP links), ifconfig must be run (somehow) each time the line is brought up or down. How this is done each time the SLIP link is brought up or down depends on the SLIP software being used. The following output shows the values for the author's subnet. Compare these values with the values in Figure 3.12. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/ip_inter.htm (15 of 19) [12/09/2001 14.46.37] Chapter 3. IP: Internet Protocol SunOS -a option says report on all interfaces sun % /usr/etc/ifconfig -a leO : flags=63<UP, BROADCAST, NOTRAILERS, RUNNING> inet 140.252.13.33 netmask ffffffe0 broadcast 140.252.13.63 slO : flags=105KUP, POINTOPOINT, RUNNING, LINKO> inet 140.252.1.29 -> 140.252.1.183 netmask ffffff00 loO: flags=49<UP,LOOPBACK,RUNNING> inet 127.0.0.1 netmask ff000000 The loopback interface (Section 2.7) is considered a network interface. Its class A address is not subnetted. Other things to notice are that trailer encapsulation (Section 2.3) is not used on the Ethernet, and that the Ethernet is capable of broadcasting, while the SLIP link is a point-to-point link. The flag LINK0 for the SLIP interface is the configuration option that enables compressed slip (CSLIP, Section 2.5). Other possible options are LINK1, which enables CSLIP if a compressed packet...
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