Tech Support Guy banner
Status
Not open for further replies.

Weird subnet mask

1K views 9 replies 5 participants last post by  Cicine 
#1 ·
Hi everyone,

I need to pick the brains of several network specialists. If you know TCP/IP like the back of your hand, please read this and see if you can help me.

A machine somewhere doesn't seem to connect to the LAN. It has a subnet mask of 255.255.204.0

Straight away, I noticed the address was subnetted, so I started working it out and it didn't make sense.

The third octet of the subnet mask concerns me. On the third octet, if you were to borrow 2 bits off the host portion (all bits set to 0) to create subnetworks, you'd end up with a subnet mask of 255.255.192.0. If you were to borrow 3 bits off the host portion, you'd end up with a subnet mask of 255.255.224.0.
So, it looked like a typo to me. They meant to put 224, not 204.
The person who set this up confirmed that this was correct but it doesn't make sense to me. You can't have a bit set to 1 after a bit set to 0 in an octet for a subnet mask, so where does the 224 come from? It should either be 192 or 224.

Am I correct or am I missing something critical here? :confused:

Cheers,

Cicine
 
See less See more
#7 ·
That is a pretty big network. Unless they are a 'large' provider who has purchased their own address space from Arin, I suspect that actually should be a /23 (255.255.254.0), which would give you something like 192.168.0.x - 192.168.1.x. Two concurrent class C's.
 
#8 ·
Hi Ddruid,

The network in question is huge, I am not talking about a private network here. It's a class A network: 10.x.x.x but the network is already subnetted in several smaller portions, ie the most common subnet mask in this network is 255.255.255.0, so 16 bits are already borrowed from the host portion....

But my question wasn't about what subnet mask the network should have or how big the network is, I just wanted to confirm that 255.255.204.0 was invalid subnet mask in general.

Cicine
 
Status
Not open for further replies.
You have insufficient privileges to reply here.
Top