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.

Link Locality

RFC 3927 describes Link-Local addressing for IPv4 communication between two hosts on a single link as follows:

A set of hosts is considered to be "on the same link", if:

-  when any host A from that set sends a packet to any other host B
   in that set, using unicast, multicast, or broadcast, the entire
   link-layer packet payload arrives unmodified, and

-  a broadcast sent over that link by any host from that set of hosts
   can be received by every other host in that set
 
The link-layer *header* may be modified, such as in Token Ring Source
Routing [802.5], but not the link-layer *payload*.  In particular, if
any device forwarding a packet modifies any part of the IP header or
IP payload then the packet is no longer considered to be on the same
link.  This means that the packet may pass through devices such as
repeaters, bridges, hubs or switches and still be considered to be on
the same link for the purpose of this document, but not through a
device such as an IP router that decrements the TTL or otherwise
modifies the IP header.