A computer on a LAN connects to the wider internet through a gateway or router computer, which
connects the LAN to the internet.
Computers communicate over a network by conforming to network protocols. Protocols are required
at more than one level. At the hardware level, the computers must use the same signaling technology, the same
medium of connection, the same speed of transmission, etc. At higher levels, the computers must agree on
what the signals mean, and when to take turns sending and receiving. One describes and implements network
protocols as multiple layers of software and hardware. The resulting set of software and hardware is often
described as the network stack.
The OSI reference model is the standard network protocol model, and it has seven layers. The internet
reference model is simpler, with four layers. For historical and pragmatic reasons, the internet model is the one
in wide use, and that is the model we described in detail.
The link level consists of the interface card and operating system driver for the physical connection between
computers. Common links today are Ethernet and Wi-Fi (wireless). Both have been standardized as IEEE standards.
The network-level protocol of the internet is IP, or internet protocol.
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