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1.9 Test Networks and Hosts

Figure 1.16 shows the various networks and hosts used in the examples throughout the text. For each host, we show the OS and the type of hardware (since some of the operating systems run on more than one type of hardware). The name within each box is the hostname that appears in the text.

The topology shown in Figure 1.16 is interesting for the sake of our examples, but the machines are largely spread out across the Internet and the physical topology becomes less interesting in practice. Instead, virtual private networks (VPNs) or secure shell (SSH) connections provide connectivity between these machines regardless of where they live physically.

Figure 1.16. Networks and hosts used for most examples in the text.


The notation "/24" indicates the number of consecutive bits starting from the leftmost bit of the address used to identify the network and subnet. Section A.4 will talk about the /n notation used today to designate subnet boundaries.

The real name of the Sun OS is SunOS 5.x and not Solaris 2.x, but everyone refers to it as Solaris, the name given to the sum of the OS and other software bundled with the base OS.

Discovering Network Topology

We show the network topology in Figure 1.16 for the hosts used for the examples throughout this text, but you may need to know your own network topology to run the examples and exercises on your own network. Although there are no current Unix standards with regard to network configuration and administration, two basic commands are provided by most Unix systems and can be used to discover some details of a network: netstat and ifconfig. Check the manual (man) pages for these commands on your system to see the details on the information that is output. Also be aware that some vendors place these commands in an administrative directory, such as /sbin or /usr/sbin, instead of the normal /usr/bin, and these directories might not be in your normal shell search path (PATH).

  1. netstat -i provides information on the interfaces. We also specify the -n flag to print numeric addresses, instead of trying to find names for the networks. This shows us the interfaces and their names.

    linux % netstat -ni
    Kernel Interface table
    eth0   1500   049211085      0      0      040540958      0      0      0 BMRU
    lo    16436   098613572      0      0      098613572      0      0      0 LRU

    The loopback interface is called lo and the Ethernet is called eth0. The next example shows a host with IPv6 support.

    freebsd % netstat -ni
    Name    Mtu Network       Address              Ipkts Ierrs    Opkts Oerrs  Coll
    hme0   1500 <Link#1>      08:00:20:a7:68:6b 29100435    35 46561488     0     0
    hme0   1500 12.106.32/24     28746630     - 46617260     -     -
    hme0   1500 fe80:1::a00:20ff:fea7:686b/64
                                                       0     -        0     -     -
    hme0   1500 3ffe:b80:1f8d:1::1/64
                              3ffe:b80:1f8d:1::1       0     -        0     -     -
    hme1   1500 <Link#2>      08:00:20:a7:68:6b    51092     0    31537     0     0
    hme1   1500 fe80:2::a00:20ff:fea7:686b/64
                                                       0     -       90     -     -
    hme1   1500 192.168.42         43584     -    24173     -     -
    hme1   1500 3ffe:b80:1f8d:2::1/64
                              3ffe:b80:1f8d:2::1      78     -        8     -     -
    lo0   16384 <Link#6>                           10198     0    10198     0     0
    lo0   16384 ::1/128       ::1                     10     -       10     -     -
    lo0   16384 fe80:6::1/64  fe80:6::1                0     -        0     -     -
    lo0   16384 127             10167     -    10167     -     -
    gif0   1280 <Link#8>                               6     0        5     0     0
    gif0   1280 3ffe:b80:3:9ad1::2/128
                              3ffe:b80:3:9ad1::2       0     -        0     -     -
    gif0   1280 fe80:8::a00:20ff:fea7:686b/64
                                                       0     -        0     -     -
  2. netstat -r shows the routing table, which is another way to determine the interfaces. We normally specify the -n flag to print numeric addresses. This also shows the IP address of the default router.

    freebad % netstat -nr
    Routing tables
    Destination        Gateway            Flags    Refs       Use  Netif Expire
    default          UGSc       10      6877   hme0
    12.106.32/24       link#1             UC          3         0   hme0        00:b0:8e:92:2c:00  UHLW        9         7   hme0   1187      08:00:20:b8:f7:e0  UHLW        0         1   hme0    140      08:00:20:a7:6e:6b  UHLW        0         2    lo0          UH          1     10167    lo0
    192.168.42         link#2             UC          2         0   hme1       08:00:20:a7:68:6b  UHLW        0        11    lo0       00:04:ac:17:bf:38  UHLW        2     24108   hme1    210
    Destination                       Gateway                        Flags      Netif Expire
    ::/96                             ::1                            UGRSc       lo0 =>
    default                           3ffe:b80:3:9ad1::1             UGSc       gif0
    ::1                               ::1                            UH          lo0
    ::ffff:                 ::1                            UGRSc       lo0
    3ffe:b80:3:9adl::1                3ffe:b80:3:9adl::2             UH         gif0
    3ffe:b80:3:9adl::2                link#8                         UHL         lo0
    3ffe:b80:1f8d::/48                lo0                            USc         lo0
    3ffe:b80:1f8d:1::/64              link#1                         UC         hme0
    3ffe:b80:lf8d:1::1                08:00:20:a7:68:6b              UHL         lo0
    3ffe:b80:lf8d:2::/64              link#2                         UC         hme1
    3ffe:b80:lf8d:2::1                08:00:20:a7:68:6b              UHL         lo0
    3ffe:b80:lf8d:2:204:acff:fe17:bf38 00:04:ac:17:bf:38             UHLW       hme1
    fe80::/10                         ::1                            UGRSc       lo0
    fe80::%hme0/64                    link#1                         UC         hme0
    fe80::a00:20ff:fea7:686b%hme0     08:00:20:a7:68:6b              UHL         lo0
    fe80::%hme1/64                    link#2                         UC         hme1
    fe80::a00:20ff:fea7:686b%hme1     08:00:20:a7:68:6b              UHL         lo0
    fe80::%lo0/64                     fe80::1%lo0                    Uc          lo0
    fe80::1%lo0                       link#6                         UHL         lo0
    fe80::%gif0/64                    link#8                         UC         gif0
    fe80::a00:20ff:fea7:686b%gif0     link#8                         UC          lo0
    ff01::/32                         ::1                            U           lo0
    ff02::/16                         ::1                            UGRS        lo0
    ff02::%hme0/32                    link#1                         UC         hme0
    ff02::%hme1/32                    link#2                         UC         hme1
    ff02::%lo0/32                     ::1                            UC          lo0
    ff02::%gif0/32                    link#8                         UC         gif0
  3. Given the interface names, we execute ifconfig to obtain the details for each interface.

    linux % ifconfig eth0
    eth0      Link encap:Ethernet  HWaddr 00:C0:9F:06:B0:E1
              inet addr:  Bcast:  Mask:
              UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
              RX packets:49214397 errors:0 dropped:0 overruns:0 frame:0
              TX packets:40543799 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:100
              RX bytes:1098069974 (1047.2 Mb)  TX bytes:3360546472 (3204.8 Mb)
              Interrupt:11 Base address:0x6000

    This shows the IP address, subnet mask, and broadcast address. The MULTICAST flag is often an indication that the host supports multicasting. Some implementations provide a -a flag, which prints information on all configured interfaces.

  4. One way to find the IP address of many hosts on the local network is to ping the broadcast address (which we found in the previous step).

    linux % ping -b
    WARNING: pinging broadcast address
    PING ( from : 56(84) bytes of data.
    64 bytes from icmp_seq=0 ttl=255 time=241 usec
    64 bytes from icmp_seq=0 ttl=255 time=2.566 msec (DUP!)
    64 bytes from icmp_seq=0 ttl=255 time=2.973 msec (DUP!)
    64 bytes from icmp_seq=0 ttl=255 time=3.089 msec (DUP!)
    64 bytes from icmp_seq=0 ttl=255 time=3.200 msec (DUP!)
    64 bytes from icmp_seq=0 ttl=255 time=3.311 msec (DUP!)
    64 bytes from icmp_seq=0 ttl=64 time=3.541 msec (DUP!)
    64 bytes from icmp_seq=0 ttl=255 time=3.636 msec (DUP!)
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