Not understanding network setup!!

vishamr2000 · 06-01-2006, 01:53 PM

Hi to all,

Code:

                 +-------+   eth1   +-------+
                 |       |==========|       |
 'network 1' ----|   A   |          |   B   |---- 'network 2'
                 |       |==========|       |
                 +-------+   eth2   +-------+

A and B are routers

# tc qdisc add dev eth1 root teql0
# tc qdisc add dev eth2 root teql0
# ip link set dev teql0 up

On router A: 

# ip addr add dev eth1 10.0.0.0/31
# ip addr add dev eth2 10.0.0.2/31
# ip addr add dev teql0 10.0.0.4/31

On router B: 

# ip addr add dev eth1 10.0.0.1/31
# ip addr add dev eth2 10.0.0.3/31
# ip addr add dev teql0 10.0.0.5/31

The above has been quoted from LARTC HOWTO. I would like to know the following things:

1) Are the 10.0.0.0/31, 10.0.0.2/31, 10.0.0.3/31...different networks? Are the devices eth1, eth2 and teql0 on each router part of 3 different networks?

2) What is the reason for doing the "/31" thing? CAn't we use network addresses such as 192.168.0.0/24, 192.168.10.0/24 and so on for the devices eth1, eth2 and teql0?

Warm regards,
Visham

iuaui · 06-01-2006, 03:38 PM

The reason doing that "/31" thing is to preserve IP addresses. But this mask gives only two IP addresses/network, so those networks have only network address and broadcast address. (This means that IP-spaces' first address is used as network address and last address is for broadcast. You can calculate it from bits left for "host" portion, meaning 32-31 = 1 bit ==> there is 2 addresses, first and last) Those network masks do not give any IP's for hosts.

My humble opinion is that /30 mask should be use for this kind of "networks". (32-30 = 2 bits which means four addresses, two for hosts and two for network and broadcast addresses)

This way using small address spaces also makes sure that ALL IP-ranges in 10.0.0.0 space are two adresses wide. Summa summarum: You have a enormous pile of small networks and you can't extend network space. All this kind of network "splitting" should be done from the tail, not from the head...