Subnet question

[jstevens13]

Ok so we have two VLANS each with their own subnet with a router connecting the them. The subnet in VLAN22 has an address range of 172.20.22.0/24 and VLAN23 has an address range of 172.20.23.0/24. So an example host in VLAN22 (let’s call this one Host A) is setup as follows: IP: 172.20.22.15, Subnet Mask: 255.255.255.0, Gateway 172.20.22.1 and an example host in VLAN23 (let’s call this one Host B) is setup as follows: IP: 172.20.23.15, Subnet Mask: 255.255.255.0, Gateway 172.20.23.1. Ok so we had a problem the other day here. Host B is setup as a webserver and Host A is running a web browser connecting to Host B. Host A is able to load pages served by Host B. No problem.

The question arose when we added a second host into VLAN22. Let’s call this one Host AA. It was incorrectly setup by the technician setting it up -- like this : IP: 172.20.22.16, Subnet Mask: 255.255.0.0, Gateway nothing. So the subnet mask was wrong (16 bit instead of 24) and there was no gateway address entered at all. However, Host AA was still able to connect to the webserver on Host B and load pages etc.
My question is, why was Host AA still able to connect to Host B on another subnet & VLAN even though it didn’t have a gateway address? I’m guessing that the answer has something to do with the incorrect 16 Bit mask on Host AA and something related to Classless Inter-Domain routing but I’m not able to offer a great explanation.

Thoughts?

 

[VinceWhirlwind]

Your network is not actually using VLANs - both subnets are in fact on the same VLAN on the switch(es) and the two devices were able to communicate at Layer 2, not requiring the router.

 

[jstevens13]

Vince,
Thanks for the answer. I was concerned that might be the case. Is that the only possible answer? There is no way what I described could happen if the two subnets were in fact in separate VLANS? Maybe some kind of other misconfiguration on the router?

 

[VinceWhirlwind]

The router doesn't come into it for two reasons:

1/ The PC has no default GW set.

2/ The PC and webserver subnet masks are telling those devices that they are both in the same Layer-2 segment.

 

[DctrCool]

What kind of managed switch are you using? Are you using port based VLAN? You might check the port the new device is connected to and make sure it is not on a trunk port.

Good luck!

Dr. Cool
STAFTLSKHO.