RE: IP Forwarding

On Tue, 13 Mar 2001, Mark Walter wrote:
> Dwayne,
> 
> I'm still a little confused. Our internal network is very simple. One 
> subnet, all with 192.168.0.xxx. I created the line description for the 10m 
> card in our AS/400. I created an IP interface in the same subnet as our 
> Internet system ex. (10.100.100.xxx subnet mask 255.255.255.224). Now, what 
> would the route description look like in order for me to use the AS/400 as 
> a gateway to the Internet?
> 
> Thanks
> 


Explanation of interfaces and routes:
----------------------------------------

1) You add a TCP/IP interface for 192.168.0.x with a netmask of
      255.255.255.0.  This is all the computer needs to know
      how to deal with anything starting with "192.168.0". 
      These IPs presumed to be directly accessible to the AS/400
      on that LAN.  

2) You add a TCP/IP interface for 10.100.100.1 with netmask
     255.255.255.224.   This means the IPs of 10.100.100.1 -
     10.100.100.30 are directly accessible on _this_ LAN, and
     packets addressed to those IPs will be sent out over
     this interface.

3) You need a default route that tells the AS/400 what to do
     with any packets that aren't on one or the other of the
     LANs that you described above.  You tell it something
     like:
 
     ADDTCPRTE RTEDEST(*DFTROUTE) SUBNETMASK(*NONE) 
               NEXTHOP(10.100.100.30)  

     The next hop has to be a machine accessible by one of
     the interfaces in #1 or #2 above.   What you're essentially
     telling it is:  "If I don't already know what to do with 
     an IP address, just forward it to 10.100.100.30."  Presumably,
     10.100.100.30 will know what to do with it.


Does that help clarify what you're doing? 

You don't need more routes than that unless you've got additional
subnets that you need to be able to get to using different routers.

Explanation of Netmask:
------------------------

Maybe it would help to understand what a netmask is?   A netmask is
just a bitmask to show what IPs are in a given subnet.   A netmask
of "255.255.255.0" when converted to binary would be: 
"11111111.11111111.11111111.00000000"

So when you use this with an IP address of "192.168.1.44" the part
represented by a "1" in the netmask is the "network" and the part
represented by a "0" is the "host".    So 192.168.1  is the network,
44 is the host.

A netmask of "255.255.255.224" is "11111111.11111111.11111111.11100000"
So the leftmost 27 bits are the network, and the rightmost 5 bits are
the host when it is applied to an IP address.

So for IP   10.100.100.17   (00001010.01100100.01100100.000 10001) 
(I put a space where the network/host parts are split)  the network
is 10.100.100.0  and the host is 17

For IP address 10.100.100.44 (00001010.01100100.01100100.001 01100)
(again, I put a space where network & host are split)  the network
is 10.100.100.32 and the host is 12.

You'll note, therefore, that 10.100.100.44 is on a different network
(or "subnet") than 10.100.100.17, with a netmask of 255.255.255.224.

As usual, I probably provided more information that you really wanted
to know. :)

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