Logic of subnetmask 255.255.255.252 networking in Packet Tracer

Question

We have an assignment to connect computers to each other using this system. I tried changing from FastEthernet to GigaEthernet and then adding this cross connection. Which our teacher says is the right way to do so.

But whenever i enter i get this

Error

Invalid IP for this subnet mask entered.

The way i try to use the logic is i cant use more than 2 or 3 IPs within 252. So interms of router IP, i try to start with 192.168.6.1 to 192.168.6.2. to lets say router 1 gig 1/0 to router 2 gig 3/0 (You cant see in the picture but its the left router middle to right router middle) and it works.

But then next is something like 192.168.6.6 to 192.168.6.7 and i get an error. What am i doing wrong?

Example of what my setup looks like interms of Cisco Packet Tracer.

Answer 1

The rule is: you cannot use the first and the last IP of a subnet.

• The first IP is reserved for the network address.

• The last IP is defined as the broadcast.

A subnet with a mask of 255.255.255.252 has 4 addresses in total, so 2 usable addreses.

If we look only at the last octet, that means:

0 - network address
1 - usable address
2 - usable address
3 - broadcast address

4 - network address
5 - usable address
6 - usable address
7 - broadcast address

8 - network address
9 - usable address
10 - usable address
11 - broadcast address

However, for a point to point link, there's an exception to the above rule: RFC3021 defining the use of /31 subnets.

Within this exception you have a subnet with 2 addresses only, both of them being usable.

192.168.1.0/31 = 192.168.1.0 & 192.168.1.1
192.168.1.2/31 = 192.168.1.2 & 192.168.1.3
192.168.1.4/31 = 192.168.1.4 & 192.168.1.5 
etc..

note that cannot simply use 2 contiguous addresses: 192.168.1.3 and 192.168.1.4 pertains to two different subnets.