Special IPv4 Addresses

 

Special IPv4 Addresses

In IPv4, some addresses are reserved for special purposes rather than being assigned normally to individual devices. The following are five important special address types.

Special address	IPv4 block/address	Main purpose
This-host address	0.0.0.0/32	Used when a host does not yet know its own IP address
Limited-broadcast address	255.255.255.255/32	Sends a packet to all devices in the local network
Loopback address	127.0.0.0/8	Testing software/networking on the same computer
Private addresses	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 169.254.0.0/16	Used for private/local networks
Multicast addresses	224.0.0.0/4	Sending data to a group of devices

Let's understand each one with simple examples.

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1. This-Host Address — 0.0.0.0/32

The address:

0.0.0.0

is called the this-host address.

It is used when a host wants to send an IP datagram but does not know its own IP address yet.

Example

Suppose a computer has just started and is trying to obtain an IP address. At this point, it does not know its own IP address.

It may use:

Source IP = 0.0.0.0

This essentially means:

"This host, whatever my current IP address is."

So, 0.0.0.0 is not normally assigned to a computer as its permanent IP address.

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2. Limited-Broadcast Address — 255.255.255.255/32

The address:

255.255.255.255

is called the limited-broadcast address.

It is used when a host wants to send a packet to all devices on the local network.

Example

Suppose a network contains:

PC1
PC2
PC3
PC4

If PC1 sends a packet to:

Destination IP = 255.255.255.255

the packet is intended for all devices on that local network.

             ┌── PC2
             │
PC1 ─────────┼── PC3
             │
             └── PC4

Destination: 255.255.255.255
              ↓
       All devices locally

Important point

Routers do not forward a packet with destination 255.255.255.255.

Therefore, the broadcast remains within the local network.

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3. Loopback Address — 127.0.0.0/8

The entire block:

127.0.0.0/8

is reserved for loopback.

A commonly used loopback address is:

127.0.0.1

When a computer sends a packet to a loopback address, the packet does not leave the computer.

It comes back to the same host.

       Computer
     ┌───────────┐
     │           │
     │Application│
     │     ↓     │
     │ 127.0.0.1 │
     │     ↓     │
     │  Loopback │
     │     ↑     │
     └───────────┘

Example

Suppose you develop a client-server application.

You can run both the client and server on the same computer:

Client
  ↓
127.0.0.1
  ↓
Server

This allows you to test the networking software without requiring two computers.

Easy way to remember

Loopback = Talk to myself

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4. Private Addresses

The following identifies four blocks as private addresses:

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16
169.254.0.0/16

These addresses are used for private/local networks.

For example, a home network might use:

Router       192.168.0.1
     |
 ┌───┼────────┐
PC1  PC2      Phone
192.168.0.2   192.168.0.3

These addresses are used inside the local network and are not used as globally unique Internet addresses in the same way as public addresses.

We later discusses their use with NAT (Network Address Translation).

Examples of private addresses

10.1.2.3
172.16.5.10
192.168.1.20

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5. Multicast Addresses — 224.0.0.0/4

The block:

224.0.0.0/4

is reserved for multicast.

Multicast means sending data to a group of receivers.

It is different from broadcast.

Broadcast

One sender → every device

          PC2
           ↑
PC1 ───────┼──→ PC3
           ↓
          PC4

Multicast

One sender → only members of a particular group

                 PC2 ✓
                ↗
PC1 ───────────→ PC3 ✓
                ↘
                 PC4 ✗

Here, PC2 and PC3 belong to the multicast group, while PC4 does not.