A.P. Lawrence

Network Routing Basics

We've seen how TCP/IP addresses combined with netmasks determine networks. For example, 192.168.200.83 and 192.168.201.35 are on separate networks if the netmask is 255.255.255.0 (a class C netmask).

If this concept isn't completely clear to you, go back and review.

See Networks 101 and Advanced TCP and CIDR

So, let's say that 192.168.200.83 is our server, and 192.168.201.35 is a Windows 95 machine. Again, it's a class C netmask (255.255.255.0), so there is going to have to be a router in the network. That router is going to have to have one port on the 192.168.200 network, and one on the 192.168.201 network.

How that happens depends on the router, but usually these things get configured initially by connecting to a serial port, and typing commands to set addresses and protocols on the other network ports. For the purposes of this article, we'll assume that the router has one port addressed at 192.168.200.1 and another at 192.168.201.1 (it's very common for routers to use the "1" address).

Starting at the Win95 machine, it needs to have a route so that it can get off its network (192.168.201) to the server's network (192.168.200). You'd do this in Settings->Control Panel ->Networks. Under the Properties for TCP/IP, you'd find a tab for Gateway. Here you'd enter the address of the router port on your network. That's 192.168.201.1 because the Win95 machine is on the 192.168.201 network.

If you dropped out to DOS and typed "ping 192.168.200.83" (that's the server address) before setting that gateway, you'd get "Destination host unreachable" (from a Unix machine you would get "No route to host"). After making this setting (and rebooting the 95 machine, you'll get "Request timed out" instead.

The problem is that the Win95 machine knows how to get to the server through the router, but the server doesn't know how to get back. The server needs a route to get to the 192.168.200 network.

So we need to do it there, also. If the server were a SCO Unix machine, we might type "route add default 192.168.200.1" (on older SCO you'd do "route add 192.168.201.0 192.168.200.1 0".

Note that we are saying our route goes to 192.168.201.0, not to a specific address: the .0 says we're giving a route to the whole network. If this were a class A network (255.0.0.0 netmask) we'd use route add 10.0.0.0, etc.

Linux is similar, but you need "gw" in there: "route add default gw 192.168.200.1"

NT is similar to Win95, though in both cases you can also issue "route add" commands at the command prompt. With NT and (I think) 95, doing this adds the routing information to the registry, so it sticks through a reboot. On SCO, that's not the case. You'll need to add the commands to a start-up file (/etc/rc2.d/S99route, for example: it doesn't exist, but you can create it) or modify the /etc/gateways file (see 'man routed'for the syntax of that file). The disadvantage of /etc/gateways is that it is used by routed, so if you are not running routed, that won't work.

You could also modify /etc/tcp and add your routes after it sets up its default routes. That has the disadvantage of modifying a system file: an upgrade will overwrite that.

SCO 5.0.4 adds a /etc/rc2.d/S90iproute script that reads /usr/internet/etc/sco_ip/routes. It's the Internet Manager that can add info to that file, but there's no reason you can't do it manually. The format is simple:

# comments are ok
# simple form
net default 10.1.1.3
# it's smart enough to delete the previous default
net default 192.168.1.2 
# routes to specific hosts
host 192.168.1.8 10.1.1.7
# netmasks optional
net 192.168.1.0 10.1.1.3 255.255.255.0
# if field 1 isn't host or net, it's ignored
happiness 172.16.80.10 10.1.1.1
sanjose 172.16.80.10 10.1.1.1

Another advantage of this script is that when called with "stop" (/etc/rc2.d/S90iproute stop), it will delete the routes listed in the file.

Beginning with 5.0.6, you can add the default route in /etc/default/tcp. Just modify the GATEWAY= line, and /etc/tcp will read that. DON'T DO THIS ON PRIOR RELEASES; /etc/tcp doesn't look for that until 5.0.6

So how does the OS know where to send a packet when you have more than one card in your machine such as when you have DS internet access and an internal network? You may not have realized this, but that's routing.

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The OS "knows" by the addresseses and netmask you have put on the cards.

Lets say you have two cards: your internal network card which is 192.168.2.10 with a mask of 255.255.255.0, and your external card which gets a DHCP address that's real, but let's pretend that right this minute it's 64.xxx,xxx.12 again with a 255.255.255.0 mask.

If you "ping 192.168.2.27" (some other machine on your internal network, the only place that packet will be sent to is your 192.168.2.10 card.

If you ping something on the network of the other card, that's where those packets will go.

So what if you ping something else like "132.xxx.xxx.89" ? That's where your default route comes into play. It might, for example, be 64.xxx,xxx.1. It *has* to be something on one of the networks defined by your cards. It could not be "144.xxx.xxx.1" for instance because you have no such network. So, if it were 64.xxx.xxx.1, the packets would go out your 64. interface and find their way to that router, which presumably can get them to the 132 address, either directly or by sending them along somewhere else.

You can see this stuff in action by using "traceroute"

Here's a traceroute from my machine to world.std.com

 1  65.96.8.1 (65.96.8.1)  12.640 ms  23.976 ms  12.765 ms
 2  65.96.8.1 (65.96.8.1)  12.103 ms  13.057 ms  12.767 ms
 3  24.128.9.177 (24.128.9.177)  12.732 ms  12.472 ms  12.177 ms
 4  SRP2-0-NDHMMA1-RTR01.necore.mediaone.net (24.91.0.55)  13.177 ms  22.586 ms  11.567 ms
 5  12.125.33.21 (12.125.33.21)  16.168 ms  14.617 ms  16.164 ms
 6  gbr1-p60.cb1ma.ip.att.net (12.123.40.138)  15.968 ms  21.932 ms  19.366 ms
 7  gbr3-p70.cb1ma.ip.att.net (12.122.5.53)  14.354 ms  16.152 ms  20.548 ms
 8  gbr4-p10.n54ny.ip.att.net (12.122.2.13)  40.957 ms  25.775 ms  19.749 ms
 9  ggr1-p370.n54ny.ip.att.net (12.123.1.125)  22.161 ms  20.915 ms  18.146 ms
10  att-gw.ny.uu.net (192.205.32.178)  49.996 ms  21.813 ms  20.946 ms
11  0.so-5-2-0.XL1.NYC8.ALTER.NET (152.63.19.54)  20.369 ms  21.435 ms  19.783 ms
12  0.so-3-0-0.XR1.NYC8.ALTER.NET (152.63.19.30)  20.199 ms  20.250 ms  20.146 ms
13  283.ATM7-0.XR1.BOS1.ALTER.NET (152.63.16.181)  30.349 ms  30.457 ms  29.981 ms
14  191.ATM7-0.GW3.BOS1.ALTER.NET (146.188.177.209)  35.961 ms  26.439 ms  40.353 ms
15  Boston-STD.std.com (192.74.137.80)  43.999 ms  61.854 ms  50.184 ms
16  world.std.com (192.74.137.5)  63.528 ms  81.948 ms  84.958 ms

See also :

Networking 101

Advanced TCP

CIDR

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