Why Do We Need Subnet Mask?

ipsubnet

Since a IPv4 address already gives the information of the network and and host, why do we still need a subnet mask?

The 1st Octet already specify the network class (1-127: A, 128-191: B, 192-223: C etc.). A, B, or C implies the number of octets for network (respectively, 255.0.0.0, 255.255.0.0, 255.255.255.0), which automatically tells you how many hosts is allowed for each class of network.

The IP gives all the information a subnet mask has, and more. Why do we need subnet mask in the first place? If the information a subnet contains is not entirely a subset of that inside an IP, then what else does it contain or what in addition does the subnet do, that require us to specify it independently?

Best Answer

We need a subnet mask for IPv4 addresses because the address doesn't give any information on the network size. Class sizes are not the network sizes. In practical networks all IPv4 networks are broken up into subnets that are smaller than the class size.

For example you could break up the class C network 200.200.200.0/255.255.255.0 into two smaller networks (potentially at separate locations) 200.200.200.0/255.255.255.128 and 200.200.200.128/255.255.255.128 assuming neither needed more than 126 hosts. In reality most companies only get enough IPv4 addresses for the servers that need to be on the public Internet. I've personally seen set ups with 32,16, & 8 address networks (that would be masks of 255.255.255.224, 255.255.255.240, & 255.255.255.248 respectively)

Having IP networks only in class size blocks was too restrictive in limited the number of networks that could be allowed - the 127 class A networks taking half of the space. Not to mention that having a 24 billion node network is completely unmanageable Instead in 1993 Classless Inter-Domain Routing (CIDR) was introduced to allow the networks to be split up.

Also to be clear the purpose of the subnet mask is to determine which hosts are on the local network and which are outside of the network. Hosts can talk directly to hosts on the same network, but they need to communicate with a router to talk to hosts on external networks.

Related Solutions

Networking – Why subnet mask for IP 10.0.1.4 is 255.255.255.0

Classful addressing has been relegated to the dustbin of history, made obsolete by VLSM and CIDR. The only place it's still used today is Cisco certification exams.

What you're seeing here is VLSM (Variable Length Subnet Mask). VLSM has been the norm on the Internet since 1995 or so after having been proposed in 1985. So your reasoning is out of date by at least 17 years.

You're also confusing subnetworks with networks. Even with classful addressing, this would be a class A network. That would mean the network mask would be 255.0.0.0. But that wouldn't say what the subnet mask was. See this link for more details.

IP Subnetting – How to Subnet 90.103.80.0/21 into 8 Subnets with 250 Hosts Each

As you already suspected it's answer 4 that's not correct. To see this it might be best to write the addresses down in binary notation.

The address range you have been given (90.103.80/21) comes down to:

01011010 01101111 01010xxx.xxxxxxxx
--------.--------.-----xxx.xxxxxxxx

with "x"es marking the part that you are free to assign any combination of "0" and "1" to.

Taking 3 additional bits as the network part you're left with the following:

01011010 01101111 01010nnn.hhhhhhhh
--------.--------.-----xxx.xxxxxxxx

With "n" marking the bits used for the network part and "h" marking the bits used for the host part.

Your networks will be thus using the combinations "000" "001" "010" "011" "100" "101" "110" and "111" to substitute the "nnn" part. Leaving (without network and broadcast adress) 254 hosts on each network with addresses distributed as follows:

01011010 01101111 01010nnn.hhhhhhhh

-- 1st network --------------------
                       000.00000000   network    90.103.80.0
                       000.00000001   1st host   90.103.80.1
                         ...           ...
                       000.11111110   last host  90.103.80.254
                       000.11111111   broadcast  90.103.80.255
-- 2nd network --------------------
                       001.00000000   network    90.103.81.0
                         ...
                       001.11111111   broadcast  90.103.81.255
-----------------------------------
          .....
-- 8th network --------------------
                         ...
                       111.11111111   broadcast  90.103.87.255

Best Answer

Related Solutions

Networking – Why subnet mask for IP 10.0.1.4 is 255.255.255.0

IP Subnetting – How to Subnet 90.103.80.0/21 into 8 Subnets with 250 Hosts Each

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