Here is a very brief overview of the seven network layers.

Layer 1 (Physical Layer)

An ethernet hub is completely in Layer 1.

Layer 2 (Data Link Layer)

An ethernet switch is partly in Layer 2 as it must have knowledge of the Media Access Control (MAC) address of packets.

Layer 3 (Network Layer)

The funtion of routers is at this level.

This is the IP layer.

Layer 4 (Transport Layer)

Ports are defined at this level.

TCP, UDP, etc reside at this level.

Layer 5 (Session Layer)

Is a conseptual layer describing sessions.

Layer 6 (Presentation Layer)

The Secure Sockets Layer (SSL) protocol is defined at this level.

HTTP compression is also handled at this level.

Layer 7 (Application Layer)

Hypertext Transfer Protocol (HTTP) resides at this level.