How a Computer Gets an IP Address?

1. The Role of the NIC (Network Interface Card)

A computer needs a NIC to connect to a network. Without a NIC, it cannot communicate and therefore has no MAC or IP address.

When a NIC is installed or enabled:

The computer gets a MAC address.
It can request and receive an IP address.

Modern laptops and desktops have a built-in NIC on the motherboard. The MAC address belongs to the NIC hardware. If you replace the NIC, the MAC address changes.

2. What happens if there is no DHCP server? (APIPA)

If a computer cannot find a DHCP server, the OS assigns a temporary IP address called APIPA (Automatic Private IP Addressing).

APIPA Range: 169.254.0.1 to 169.254.255.254.
APIPA is used only when DHCP fails.

3. Understanding the Router: LAN vs. WAN

A router has two main interfaces:

LAN Interface

Connects local devices (computers, phones, etc.)
Has a private IP address (e.g., 192.168.1.1)
Acts as the default gateway

WAN Interface

Connects to the Internet Service Provider (ISP)
Receives a public IP address from the ISP

Most home routers:

Have a pre-configured LAN IP address
Automatically obtain the WAN IP from the ISP

4. What Happens When You Connect a Computer to a Router?

When you connect a computer to a router using an Ethernet cable:

The NIC becomes active.
The computer sends a broadcast message asking:"Is there any DHCP server available?"

Even if the computer does not yet know the router’s IP or MAC address, it can still send this message using a broadcast.

5. How Does the Computer Get an IP? (DORA Process)

Most routers run a DHCP (Dynamic Host Configuration Protocol) server.When a computer connects to a router, it gets an IP using the DORA process:

D – Discovery (Computer broadcasts for DHCP server)
O – Offer (Router offers an IP)
R – Request (Computer requests that IP)
A – Acknowledge (Router confirms)

Now the computer has:

IP address
Subnet mask
Default gateway
DNS server