Internal and External IP Addresses Explained


Updated: December 4, 2016

You may not know it, but you have two IP addresses, an Internal IP address and an external IP address.

Why do you need two IP  addresses and what is the difference between them?

Finding Your External and Internal IP Addresses

To find your external IP address do a search for your IP address in Google then you will probably see a screen like this

whatismyipaddress-google

Google shows an IP address:

109.155.209.167

This is your external IP address.

Now  open a command line and type{

ipconfig

then I see something like this:

myipaddress

This time your IP address is 192.168.1.64

This is your Internal IP address.

The internal IP address,  is used on your local internal network and the external IP address is used when communicating with machines on the Internet.

Note: The terms private (internal) and public (external) addresses are also used.

The reason why you seem to have two IP addresses is because IPv4 has a limited amount of IP addresses, and to preserve IP addresses a techniques called NAT (network address translation) is used.

NAT allows thousands of computers on an internal network access the Internet using a single external IP address.

The internal IP addresses use a special IP address range. This address range is specifically reserved for internal addresses and the IP addresses will not be forwarded by routers on the Internet.

All of the devices on my home network have addresses like 192.168.1.x but externally they have the address 109.155.209.167

Here is a diagram to illustrate the IP address allocation on a typical home or small business network.

internal-external-IP-addresses

Home networks usually use addresses in the 168.192.0.* or 168.192.1.* address range.

The 168.192.0.0 network address is a designated non route-able private network address.

However they are not the only non route-able private address ranges that you can use. Others address ranges that can be used are:

  • 10.x.x.x is a 24 bit address block
  • 172.16.x.x to 172.31.x.x is a 20 bit address block.

See wiki private networks for more details

Note: You may want to read the IP4 Addresses article if you are unfamiliar with network addresses.

Allowing Access from The Internet

Because of the NAT router there is no direct connection between the Internet, and a computer on the local network.

All communication between your Internal network and the Internet must be initiated by a device on your Internal network.

Generally this isn’t a problem, and is a security advantage.

However online gamers, and those who would like to host a website or other services on their own network will need to allow external devices on the Internet to access them.

To accomplish this NAT routers can be configured to use a technique called port forwarding.

Port forwarding allows an internal device to to appear to have an external IP address, and allows incoming connections from the Internet.

Summary

Home and business networks use private or internal addresses from a reserved non route-able address range,

A NAT router connects the internal network to the Internet using a single route-able external or public IP address assigned by the ISP.

The NAT router allows connections to be established only from the internal network to the external network and not vice-versa unless techniques like port forwarding are employed.

Common Questions and Answers

Q-My External IP Address Keeps Changing. Is this normal? can I stop it?

A- Most ISPs will allocate dynamic IP addresses to their customers. This Address will become the External IP address assigned to your router, and hence it could change.

This is a screenshot showing how my own external IP address has changed over a month.

external-ip-address-changes

Some ISPs will allow you to have a static IP address but usually at a premium ( Business broadband)

Static IP addresses do not change.

If you intend providing access to your network from the Internet, then you really need a static external IP address.

However an alternative for home and small business networks is to use a dynamic DNS service.

A dynamic DNS service doesn’t stop the external address from changing but instead it tracks these changes automatically and updates the DNS records automatically when the IP address changes.

Q- Can You Use an internal IP address on the Internet?

A- No – The internal IP address ranges discussed above are blocked by Internet routers.

Q- How does a message reach an Internal IP Address from an external one?

A- This is done using a translation table, and is covered in understanding port forwarding.

Q- Does in make any difference if I use the 10. IP address range and not the 192.168.0.0 address range.

A – No none at all. However most home routers are preset to use the 192.168 range.

Q- Who assigns my external IP address?

A- It is assigned by your ISP.

Q- How do I know if my External Address has changed?

A- You will need to check it using the technique shown earlier.

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