Public, private, static and dynamic IP addresses

By the end of this lesson

By the end you can explain the difference between public and private IP addresses (and why it matters for security), compare static and dynamic addresses, and describe how a device gets its IP address.

Millions of homes, one address each

Your laptop is probably 192.168.1.something. So is your neighbour's. So is a laptop in Lima and one in Lahore.

That works because those are private addresses: they only have to be unique inside each home. To the rest of the internet, each home shows a single public address.

Definition

Public IP address

An IP address that is unique across the whole internet, allocated by an ISP, and can be reached directly from anywhere on the internet.

Definition

Private IP address

An IP address used inside a LAN. It only has to be unique within that network, and it can't be routed across the internet. Private addresses come from reserved ranges:

10.0.0.0 to 10.255.255.255
172.16.0.0 to 172.31.255.255
192.168.0.0 to 192.168.255.255

Inside: private addresses. Outside: one public address. The router translates between them.

How private devices reach the internet

The router uses NAT (network address translation). When a packet leaves the LAN, the router replaces the private source address with its own public address and records which device sent it. When the reply comes back, it looks up that record and passes the reply to the right device.

The security implications

A device with a private address can't be contacted directly from the internet. Outsiders only see the router's public address, so the devices behind it are hidden. That's a big protection.
A device with a public address is visible and reachable by anyone on the internet, so it's more exposed to attack and needs a firewall and careful security.

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

Multiple choiceQuestion 3 · Apply

Answer the 3 questions above to continue.