The small office network

The typical medical office will have a network. Usually this will be a Windows Active Directory network with a domain controller.

It is useful to consider networks at various layers:

  • Layer 1 – the physical network – cat6 cabling and/or wireless (bits)
  • Layer 2 – the data-link layer – ethernet or 802.1 wireless (frames)
  • Layer 3 – the network layer – IP (packets)
  • Layer 4 – the transport layer – TCP, UDP etc

Each device (printer, computer, network switch etc) has a Layer 2 address (the MAC address) and a layer 3 address (the IP address). The MAC address is usually set by the manufacturer and the IP address is set by the user as part of the network setup

All devices connected together at layer 2 form what is known as the broadcast domain. Typically this would be all of the devices on the office network. They are linked together by a layer 2 switch or wireless access point

Every IP address (layer 3) is a member of a subnet. The subnet is defined by a combination of the IP address and the subnet mask. A typical office network has a maximum of 254 IP addresses on a subnet for example :

the addresses between and with a subnet of

If a device tries to communicate with one at a different subnet – for example the Google DNS server at the layer 2 frame needs to be sent to a device within the subnet that can forward the layer 3 packet out of the network towards the destination. That device is known as the gateway or router.