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In my understanding, the net-id, subnet-id and host-id of a host IP address are portions of it, indicating which bits in its binary notation they corresponds to.

From them we can evaluate the network address and subnet address, which instead are IP addresses.

The converse is not true, unless you specify the subnet mask or CIDR.

In this way it does not make sense to specify the subnet-id with a single decimal number, f.i.

in 46.179.50.44/13 the subnet-id is the 5 most significant bits of the second byte, but if I specify the subnet address 46.176.0.0 without the CIDR or subnet mask I can't get the subnet-id. "176" does not specifies the subnet-id as 46.176.0.0/13 and 46.176.0.0/15 have the same "176" but different subnet-id's.

Is that correct? I see that in some textbooks the terms are used interchangeably.

  • Because classful networking is dead, the terms network and subnet are used interchangeably. Classful networks were the network, and a subnet was anything that divided a classful network. Classes no longer exist, so a network is what you use today. Many people simply cannot let go, so the terms get used. You can subnet any network, but the subnets are now networks, too. – Ron Maupin Nov 5 '16 at 14:45
  • I get it that classful networking is dead. But let's suppose you have to explain this subject to a student (starting with classful networking, then subnetting, then vlsm and so on) you want to give the correct definition. So, would you say that 176 is the subnet-id or would you distinguish between the concept of subnet-id and the subnet address? – Nicola Nov 5 '16 at 16:26
  • Classful networking shouldn't be taught anymore. If it is necessary to teach it for historical purposes, it should be taught after a student has mastered subnetting, not before, because it colors and confuses subnetting. – Ron Maupin Nov 5 '16 at 16:29
  • And in its original meaning, subnet-id and subnet address were the same concept? – Nicola Nov 5 '16 at 16:57
  • That is probably true, but it may depend on the context. You may also say a prefix is the same thing. The context can play a big part in the specific meanings of some terms. – Ron Maupin Nov 5 '16 at 17:00
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Subnet ids are a feature of classful addressing, which was obsolete before you were born. In classless address (CIDR) the subnet mask divides the network and host ids.

Also remember that IP addresses are simply 32 bit binary numbers. The dotted decimal notation is just to make it easier for humans to read. The dots have no significance.

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