I'm studying about class less inter domain routing.

I've come to know that all zeroes (0) in host id bits refers to subnet address and all ones (1) in host id bits refers to broadcast address. So, these are reserved. e But why all zeros or all ones in subnet id bits reserved?

May be similar reason of all 0s or all 1s in host ids?


Let, domain address is:
Subnet mask is: 11111111 11111111 11111111 11000000
Domain address in binary (bold bits are subnet bits): 11001000 11110001 01110000 00000000
My reading material says, 2^5 - 2 = 30 subnets are possible only. 2 gets subtracted for two reserved subnet ids.

  • I actually discussed the fact that outdated material and classes refer to this practice in the second part of this two-part answer.
    – Ron Maupin
    Dec 27, 2018 at 14:22

1 Answer 1


The material to which you refer is simply wrong and outdated. RFC 1878, Variable Length Subnet Table For IPv4 (December 1995) explains it:

Subnets and Networks

The number of available network and host addresses are derived from the number of bits used for subnet masking. The tables below depict the number of subnetting bits and the resulting network, broadcast address, and host addresses. Please note that all-zeros and all-ones subnets are included in Tables 1-1 and 1-2 per the current, standards-based practice for using all definable subnets [4].

Your (incorrect) reading material is really mixing up CIDR (Classless Inter-Domain Routing) with an old practice from classful addressing. Under classful addressing, a subnet was a network with a mask longer than the natural class mask, and all the subnets of a classful network had to be the same size.

Under CIDR (RFCs 1517, 1518, and 1519 in the year 1993), there are no more network classes, and every network is a subnet of The subnets of a larger network can be variable sizes, and it is impossible to determine what would be the all-zeroes and all-ones subnets of a variably subnetted network because the all-zeros and all-ones subnets could be further subnetted.

The practice of reserving the all-zeroes and all-ones subnets died with network classes.

The term subnet really does not mean what it used to mean. It used to mean a network with a longer mask that the natural mask of the class. Under CIDR and VLSM, subnet is usually a verb meaning to divide a network of any size into larger networks (smaller hosts), or it is often used to refer to a network derived from a smaller network (larger hosts). Today, a network of any size is really just a network because all networks are subnets of the network.

  • Thanks.Just another question. In my material, it talks about doing CIDR using Fixed Length Subnet Mask Addressing Scheme. What I've learned about Fixed Length Subnet Mask addressing from there matches with your definition of classful addressing. So, is Fixed Length Subnet Masking falls under CIDR or classless ? Or, let me know if fixed length subnet masking is a different thing
    – Utshaw
    Dec 28, 2018 at 11:23
  • You could certainly subnet a network into subnets of all the same size. Subnets of the same and different sizes are discussed in the answer in my comment above that links to it.
    – Ron Maupin
    Dec 28, 2018 at 17:47

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