IPv4 Router – Why is This a Network if All Host Bits Are Zeros?

ipv4router

let's say that we have a router with address: 172.31.219.33/27 from this I can tell several things:

Subnet mask: 255.255.255.224
Broadcast address (here I am not sure if last digit should 64 or 65 – see later): 172.31.219.64
Last host address (since we know broadcast address; also not sure if last digit should be 63 or 62): 172.31.219.63

We could also tell first host address (172.31.219.33) and network address (172.31.219.32).

I don't understand two things:

Why is first host address X.X.X.33? It is like the router is already host?
Why is network address X.X.X.32? In other way, why it is not X.X.X.33 (from router address)?

Best Answer

Why is first host address X.X.X.33?

It is like the router is already host?

Why is network address X.X.X.32? In other way, why it is not X.X.X.33 (from router address)?

The reason for calculating network addresses and host addresses like this is specified in RFC 950 and RFC 1878; contiguous binary network masks are used to divide IPv4 addresses into host portions and network portions; if the host portion is all-zeros, that is by-definition a network, and not a host.

10101100.00011111.11011011.00100001    => IP 172.31.219.33 as binary
11111111.11111111.11111111.11100000    => 27-bit network mask (binary AND with IP for net)
                              ^^^^^
                                Host bits are zero in a network mask
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
             Network bits are ones in a network mask
----------------------------------------------------------------------------
10101100.00011111.11011011.00100000    <= 172.31.219.32 is the *network*

If you need help understanding how to calculate these correctly, we have many Q&As which already respond to this problem.

Related Solutions

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.

Ip – Subnet masks vs. IP address ranges

Your idea of start and end addresses will slow routing decisions down a lot, and a packet may traverse multiple routers to get to its destination; mask and compare for equality uses far fewer CPU cycles than compare for >= AND <=. More CPU cycles equals more time and more CPU load. Using a mask is fast and easy to do in either software or special hardware (ASIC). You have to remember that routers switch packets individually, and a router must make the determination which way to switch a packet for each and every packet which passes through it, and each router through which a packet travels will need to do the same thing.

How can route aggregation work with odd sized subnets? The number of Internet routes is already out of hand, and the ISPs won't advertise any IPv4 prefix longer than /24.

Your idea adds unacceptable overhead and delay to packet switching. You also need to think about so tightly restricting subnets that there is no room for growth. If you have assigned 192.168.1.1 to 192.168.1.17 for subnet 1, and each subsequent subnet is equally restrictive, what happens when a manager on subnet 1 wants to add a printer, scanner, new PC, etc. Suddenly, you need to move everyone to a new, larger subnet, and you have an unused subnet that may never have a good fit, thereby wasting those 17 addresses.

This idea doesn't really solve any problems since even doubling the number of available IPv4 addresses will not even come close to fixing the IPv4 address shortage, and it adds considerably to router overhead and real world implementation support.

Keep thinking about things, but realize that there are many, many extremely intelligent people who live, eat, breathe, and sleep this stuff, and have for many years, with no thought but how to make it work better. Everything about IP is under constant review and can be changed if a consensus decides it should be. The proof is the demise of IP classes in favor of CIDR and VLSM.

Best Answer

Related Solutions

How is the IP prefix passed to the router

Ip – Subnet masks vs. IP address ranges

Related Topic