Layer 3 — Network

What This Layer Does

MAC addresses work beautifully inside one local network and are useless outside it. Nothing in a MAC address tells you where the device is, so there is no way to work out a route to it. Reaching a server on the other side of the world needs a different kind of address and a different kind of device.

That is the Network layer. Its job is to move data between separate networks. The unit it works with is the packet: the data plus a Layer 3 header holding a source IP address and a destination IP address.

An IP address is a numbered location on a network — something like 192.168.1.42 or 203.0.113.7. Unlike a MAC address, it is structured. Part of it identifies the network, and part of it identifies a device on that network. That structure is what makes routing possible: a device can look at a destination address and tell whether it belongs to the local network or to a distant one, without knowing anything about the distant one.

Moving packets between networks is done by routers. A router connects two or more networks. When a packet arrives, the router reads the destination IP address, consults its routing table to decide which connection leads toward that network, and forwards the packet out of it. The next router does the same. No single router knows the whole path; each knows only the next step.

Your own device does a small version of this every time it sends anything. If the destination is local, it delivers locally using Layer 2. If it is not, it sends the packet to its default gateway — the router that handles everything not local. At home, that is the box from your internet provider.

Common Examples

• IPv4, the original addressing scheme, written as four numbers such as 192.168.1.42
• IPv6, the newer scheme with far more available addresses, written in hexadecimal such as 2001:db8::1
• Routers, including the one in your home
• The default gateway setting on every device
• ICMP, the protocol behind the ping command, used to report problems and test reachability

Two things to note and then set aside. Subnetting: because an IP address has a network part and a device part, administrators can split one large address range into smaller networks by moving where the boundary falls. That is the whole idea; the arithmetic belongs in another book. And IP delivery is described in the standards as best effort — Layer 3 tries to deliver each packet and promises nothing. Guarantees come from Layer 4.

Simple Real-World Example

A laptop at home requests a page from a website. The website's server is on a different network entirely.

The laptop compares the destination IP address with its own network and sees that it is not local. It builds a packet addressed to the server and hands it to its default gateway, the home router. The home router forwards it toward the internet provider, whose routers pass it on, each choosing the next step, until the packet reaches the network the server sits on.

  Home network                                  Server network
  ┌──────────┐    ┌────────┐   ┌────────┐   ┌────────┐   ┌────────┐
  │  Laptop  │───>│ Router │──>│ Router │──>│ Router │──>│ Server │
  └──────────┘    └────────┘   └────────┘   └────────┘   └────────┘
              each router reads the destination IP address
              and chooses only the next step

What Can Go Wrong?

• No IP address, or the wrong one. A device with an address on the wrong network can reach nothing useful, even with a perfect cable.
• The default gateway is missing or wrong. Local devices work, everything on the internet does not.
• A route is missing or broken. Packets travel partway and are dropped at a router that has no way forward.
• Duplicate IP addresses. Two devices claiming the same address makes delivery unpredictable for both.

Remember This

Layer 3 uses IP addresses and routers to move packets between networks.