Net/Subnet-ID vs Network/Subnet Address – Definitions Clarified

ipv4

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.

Best Answer

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.

Related Solutions

Subnetting aggregation

Address:   192.168.12.0         11000000.10101000.0000110 0.00000000
Netmask:   255.255.254.0 = 23   11111111.11111111.1111111 0.00000000
Network:   192.168.12.0/23      11000000.10101000.0000110 0.00000000
HostMin:   192.168.12.1         11000000.10101000.0000110 0.00000001
HostMax:   192.168.13.254       11000000.10101000.0000110 1.11111110
Broadcast: 192.168.13.255       11000000.10101000.0000110 1.11111111
Hosts/Net: 510                   Class C, Private Internet

Here you go. First you take the network id (192.168.12.0), and the mask (23) says the first 23 bits are static, and the remaining 9 bits are used in your network.

So in your case:

11000000.10101000.0000110 0.00000000
First 23 bits are to here^

Remaining 9 bits go from all zeros to all ones

If you write the IP back to decimal form, you get IPs from 192.168.12.0 (last 9 bits are zeros), to 192.168.13.255 (last 9 bits are all ones). First 23 bits are unchanged.

Since 9 bits are used for host IPs, thats 2^9 = 512 IPs (minus one for network ID and minus one for the broadcast address = 510 usable IPs).

How is the IP prefix passed to the router

How do routing protocols carry the prefixes? Where is the "slash" part stored?

I'll answer this as direct as possible with OSPF. Below is a Type 1 Hello packet sent out all interfaces to form an adjacency.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Version #   |       1       |         Packet length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Router ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Area ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |             AuType            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Authentication                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Authentication                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Network Mask                           |    <------
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         HelloInterval         |    Options    |    Rtr Pri    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     RouterDeadInterval                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Designated Router                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Backup Designated Router                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Neighbor                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |

^{RFC 2328, A.3.2 The Hello packet}

The Network Mask field is populated with the subnet mask you're using with that interface.

CIDR Notation is nothing more than a shorter version of your subnet mask for the sake of brevity. And for that matter, subnet masks are just a condensed version of bit positions. Imagine having to write in decimal every time you wanted to configure a port.

11111111.11111111.11111111.00000000    <---- This would be painstaking
255.255.255.0                          <---- Better, but not optimal
0xffffff00                             <---- Shorter still, but still not optimal
/24                                    <---- Best-case scenario

Everything comes out to the same thing in the end. So having these shorthand ways of representing the same data just makes sense.

Best Answer

Related Solutions

Subnetting aggregation

How is the IP prefix passed to the router

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