Subnetting classless network

Hello,

I am sure this is fairly basic but after searching extensively I just can' t seem to work it out.

My understanding is that classless networks can use any number of bits for the network portion eg 10.1.1.0/15 would use 15 bits for the network portion, 10.1.1.0/23 would use 23 bits etc.

What I don't understand is if I have a IP address of 10.1.1.0/27 in a classless network how can I work out the number of possible subnets?

If the network portion of the ip address can be any length how can I tell which of the 27 bits belongs to the subnet id? eg the network could be specified in the first 24 bits which would leave 3 bits for the subnet id or the network id could be the first 17 bits leaving 10 for the subnet id.

Until now I have been looking at the class of the IP adress and using that to work out the number of network and subnet bits eg 192.1.1.1/26 , as it is class C 24 of the bits belong to the network id and the remaining 2 in the subnet mask (26-24=2) are used for the subnet ID.

Thanks in advance for any help

Hi,

What I don't understand is if I have a IP address of 10.1.1.0/27 in a classless network how can I work out the number of possible subnets?

Ans:

1. Default network bits for Class A address is 8 Bits and Host is 24 Bits.

2. In your case you had borrowed 19 Bits from the host that is 8+19 = 27 bits.

3. Total number of achievable subnets is 2^19 - 2 = 524,286 Network ID's you can attain. so the remaining 5 bits(32-27) will be the host ID's and 2^5 - 2 = 30 Hosts ID's you can attain.

If the network portion of the ip address can be any length how can I tell which of the 27 bits belongs to the subnet id? eg the network could be specified in the first 24 bits which would leave 3 bits for the subnet id or the network id could be the first 17 bits leaving 10 for the subnet id.

Ans: Default Thumb rule

1. Class A : 11111111.00000000.00000000.00000000 = 1 is Network ID and 0 is host ID, 8 One's = 8 bits Network ID's and 24 Zero's = 24 bits Host ID's.
2. Class B : 11111111.11111111.00000000.00000000 = 1 is Network ID and 0 is host ID, 16 One's = 16 bits Network ID's and 16 Zero's = 16 bits Host ID's
3. Class C : 11111111.11111111.11111111.00000000 = 1 is Network ID and 0 is host ID, 24 One's = 24 bits Network ID's and 8 Zero's = 8 bits Host ID's

4. Range of IP Addresses.

Class A = 1- 126 Class B = 128 - 191 Class C = 192 - 223 Class D & Class E for Multicast Addresses.

Hope the above explanation clarifies your doubt. However if you have any doubts Feel free to ask any questions.

Regards,

Rizin

Hello,

So If I understand correctly when we don't know the network ID the rule of thumb is to go by classes to work out which bits belong to the network ID.

If I actually had the network address eg 10.1.1.0/16 (which is possible with classless, correct?) and I had a host IP address 10.1.1.0/27 would this be correct:

27-16=11
2^11 -2=2046 subnets available

I'm guessing the above is how it is normally done in the workplace/home network as you would be able to lookup the network address.

Thanks for the help

Hello,

If I actually had the network address eg 10.1.1.0/16 (which is possible with classless, correct?) and I had a host IP address 10.1.1.0/27 would this be correct: 27-16=11 -> 2^11 -2=2046 subnets available.

Answer: No.

Let me put in other way. When you say /16 it means it belongs to one block, when you say /20 it means it belongs to other block, when you say /27, it means it is belongs to other distant block, and so forth...

Following example defined here for Class A adddress:

(i). When you say /16 it means, from the total number of 32 Bit Address Class A Default Network ID's is 8 bits and you borrowed 8 bits from the host id's to make it 16 bits.
(ii). However here you borrow 8 bits from the default host id's.
(iii). So here you always only calculate the borrowed bits network ID's or subnet ID's.
(iv). So given this scenario you can achieve 2^8 - 2 = 254 Network ID's or 256 Network ID's if your router supports subnet zero, and remaining 16 -> 2^16 - 2 = 65,534 Host ID's you can achieve per subnet.

In your example you have 10.1.1.0 /16:

It means it starts as 10.1.0.0 is your First Subnet ID or Network ID from the total number of 254 or 256 refer above.

A. if you do not configure subnet zero, then following is the answer.

1. First Network ID = 10.1.0.0 First Host ID = 10.1.0.1 Last Host ID = 10.1.255.254 Broadcast ID = 10.1.255.255.

2. 10.1.1.0/16 is belongs to the number of 256th PC, Node or Host.

3. 10.1.1.0/27 is your imaginary address isn't it ?

I strongly suggest that you should read more and hard work on the Subnet subjects and i hope above explanation clarifies your doubts, please feel free to ask any questions at any time.

Regards,

Rizin