Subnetting

[Anmol7]

I was going through subnetting. I turn into a doubt.

As per books and tutors.

ClassA can have mask from /8 to /32 and have variable subnets and same for next two classes

Class B can have from /16 to /32

Class C can have from /24 to /32

My question is , Can a class C network have /9 or /14 mask, if yes then how many number of subnets for it ?

[Cristian Matei · Accepted solution]

Hi,

    Yes you can do that, having a /9, or /6 or a /4 for a class A, or class B, or class C address. This is called supernetting or CIDR (classless inter domain routing). The number of available subnets follows the same logic as in subnetting, cause in the end from a /4 you may need multiple /8 subnets. So number of available subnets, is based on how many bits you borrow for subnetting, while the remaining bits give you the number of hosts/IP's available within each subnet.

Regards,

Cristian Matei.

[Joseph W. Doherty]

"Yes you can do that, having a /9, or /6 or a /4 for a class A, or class B, or class C address."

Wouldn't a /9 be a subnet of a class A?

[Anmol7]

Thanks Cristian

[Joseph W. Doherty]

"My question is , Can a class C network have /9 or /14 mask, if yes then how many number of subnets for it ?"

Yes and none, because when you go larger than the Class address block (as Christian notes), you're supernetting, not subnetting.

For example, if you take a Class C, and supernet to a /9, and then decide to split it into two equal "subnets", i.e. two /10s, then you have two Class C supernets where before you had but one.

In any case, when splitting or joining address blocks, it's always in power of twos. So, the prior /9 could be split into two /10s, which one, or both of the /10s could be split into two /11s, which a /11 can be split into two /12 and so on.

However, when joining networks, two /12 cannot become a single /11 just because they are adjacent. Then can only be joined if the two /12s would be spit from the same /11.

For example 192.168.5.0/24 can be split into 192.168.5.0/25 and 192.168.5.128/25 but 192.168.4.128/25 cannot be joined with 192.168.5.0/25 into a /24.