General discussion

Locked

Class C Subnet Mask

By tonyw ·
Is it possible to use a subnet mask of 255.255.0.0 on a 192.x.x.x network? This seems to be illegal to me but can someone confirm one way or the other please?

This conversation is currently closed to new comments.

9 total posts (Page 1 of 1)  
| Thread display: Collapse - | Expand +

All Comments

Collapse -

by Jaqui In reply to Class C Subnet Mask

that is actually the standard practice.
mask with 255.255.255.255

Collapse -

by tonyw In reply to

I don't understand the answer?? Surely 192. is a class C address and the default mask is 255.255.255.0

Collapse -

by CG IT In reply to Class C Subnet Mask

RFC 1918

the 255.255.0.0 is a Class B where the first 2 octets define the network portion of the address and 3rd and 4th octets defines the host.

Class C the first 3 octets define the network portion and the 4th octet defines the hosts of the address.

The subnet mask masks the network portion of the IP address so that only the host portion is visible.

You can use 191.xxx.xxx.xxx for class B

Remember that the address in decimal form represents the address in binary form. Also remember that the Class of addresses determines the # of hosts available on the subnet and the # of networks available. Class A has the lest # of hosts. Class B had the more and Class C has the most # of hosts available per subnet.

Collapse -

by tonyw In reply to

I understand all that. You still haven't answered the question of wether it is a valid address and mask or not?
Can I have an address of 192.x.x.x with a subnet mask of 255.255.255.0 ?

Collapse -

by mark.hodgson In reply to Class C Subnet Mask

Do you really mean a 192.168.x.x private network? IANA has reserved three IP address blocks for private use:

10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

So you CAN use the subnet mask 255.255.0.0 with a 192.168.0.0 network, but it is awfully wasteful! There is no reason why you should not use subnetting to use just a portion of this available address space at a time, ideally using 24-bit masks, e.g.

192.168.0.0/24 (SNM=255.255.255.0)
192.168.1.0/24

Remember that your subnet mask can be any length from 16 to 32 bits for this address range, so it really depends upon how many hosts you anticipate on your private subnet. Any less than 253, you should stick to a 24-bit mask to make life easier.

Finally, do not get hung up on IP address classes - they have been mostly superceded by classless addressing, to make the best use of available IP address blocks, particularly with public IP addresses.

Collapse -

by tonyw In reply to
Collapse -

by GDF In reply to Class C Subnet Mask

The previous answer is very good, but I wanted to add a couple of things. First, there are organizations (and ours is one) that own an entire block of 192.N.xxx.xxx Class-C addresses. We could theoretically use that as though it were a single class-B subnet with a 255.255.0.0 subnet mask.

The larger the subnet, the more nodes that can talk to each other without going through a router. That could be seen as an advantage in the old days of TCP/IP, but it's not very important now. The down side of using such a large single subnet is that any broadcast packets have to be propagated to (and seen and processed by) that large node population.

I would again point you to the previous response, in particular the information about the 192.168.xxx.xxx private subnet space. I have often seen net admins use a class B mask in that space, even though it is not typically useful or warranted.

Collapse -

by tonyw In reply to

Thank you for the extra detail

Collapse -

by tonyw In reply to Class C Subnet Mask

This question was closed by the author

Back to Desktop Forum
9 total posts (Page 1 of 1)  

Related Discussions

Related Forums