Network2/Concept/Link

When talking about networking, a link is what you get when you associate (connect) a medium with a network interface. In this case a medium is something you can send signals over, such as a network cable or a specific frequency in the electromagnetic spectrum for a wireless networking card. Links become interesting when there are 2 or more interfaces attached to them, which typically happens through bridging several mediums. Switches and hubs serve this role in wired networks, as do Access Points for wireless networks. Links encompass "media access disciplines", called "media access control protocols" (commonly abbreviated as MAC) which operate at a level below that of IP.

Ethernet is a common link-layer protocol which commonly carries (fragmented) IPv4 or IPv6 packets, among other things. Ethernet transmits data in frames, which are often smaller than IP packets. When this happens the packets are fragmented, with each ethernet frame having a complete IP header and fragment index.

So far the links we have been talking about have been built on top of 'physical' mediums; they exist very low in the OSI stack. Links can also be created on tunnels, which present 'media' built from higher level protocols. VPNs are an example of this.