Newtorking and routing explained [??]

["Robert G. Brown" <bob@xxxxxxx>]

Sorry I didn't get back to you last night with an explanation of the
routing table, but I had a couple of things that came up which had to be
dealt with.

Anyway, I can't figure out why this has so many entries! Chalk it up to
the excess of Microsoft or something. But I'll hit the rows I recognize.

Row 1: This is what gets you onto the Internet. The network address is
0.0.0.0, also known a "default". Packets that are addressed to IP
addresses outside of your LAN will be sent to the IP address specified in
the "Gateway Address" column on this row... THAT adress is the address of
the machine you hooked up to by dialing up your ISP.

Row 2: No Idea

Row 3: This routing brings stuff down from your ISP to your machine.

Row 4: I don't know, but I THINK it includes your machine in IP
       multicasting from your ISP.

Row 5: This is the localhost loopback routing.

Row 6: This routing puts out any packets from your gateway box onto any
       and all machines on your LAN.

Row 7: Routes from the interface (your ethernet card?) to the localhost 
       IMO not needed, but it can't hurt! :)

Rows 8: Provides a broadcast interface for your LAN

Row 9,10: Don't know... isn't nessesary AFAIK. Might be something your ISP
          defines.to authenticate the connection.

Row 11: Don't really know, may be the ethernet card interface definition

Below that, you show two connections (other commands show this on UNIX
systems). One is to AOL, and the other is to the spinway.com [??] web
server.

Here (in contrast) are the routing tables from two hosts on the LAN here:

zeppo
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.24.0    *               255.255.255.0   U     0      0        0 eth0
127.0.0.0       *               255.0.0.0       U     0      0        0 lo
default         chico           0.0.0.0         UG    0      0        0 eth0

This is a typical workstation routing table. The first line routes to ANY
other station on the same LAN (192.168.24.x). The second is the loopback
interface, and the third routes anything ELSE to chico.

Here, I used an option of route that shows the ip addresses, not the
hostnames. default is replaced by 0.0.0.0... so let's look at it:

chico
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
206.31.33.23    0.0.0.0         255.255.255.255 UH    0      0        0 ppp0
192.168.24.0    0.0.0.0         255.255.255.0   U     0      0     3827 eth0
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        3 lo
0.0.0.0         206.31.33.23    0.0.0.0         UG    0      0      121 ppp0

The first line establishes the Internet connection; note that the name of
the interface is ppp0 (over on the right). The second line is just like
the the first line of the table from zeppo, it allows chico to reach all
the LAN workstations. The third line is the loopback interface. The last
line is the punchline; it routes all the packets that don't belong HERE
out to the machine I'm dialed up to (206.31.33.23).

Where MY Internet address? It's not shown in the routing table, but it's a
property of the interface ppp0, and we can see it like this:

# ifconfig ppp0

ppp0      Link encap:Point-to-Point Protocol  
          inet addr:208.137.51.109  P-t-P:206.31.33.23  Mask:255.255.255.0
          UP POINTOPOINT RUNNING  MTU:1500  Metric:1
          RX packets:6449 errors:0 dropped:0 overruns:0
          TX packets:7444 errors:0 dropped:0 overruns:0

See? I'm 208.137.51.109, and I'm hooked up to 206.31.33.23... so anything
that flows in from there comes through ppp0 and gets routed (or firewalled)
and this is the meaning of the first row. Simialrly, any packet chico gets
and doesn't know what to do with will be routed to 206.31.33.23 because
of the LAST row of the table. I'll leave it to you to identify the same
rows in your routing table.

Hope this helps!

                                                               Regards,
                                                              ---> RGB <---