This page is a simple introduction to the concepts involved in understanding addresses on local networks. If you are looking for information about how your internet connection fits in with your network, please see Public IP addresses.
Each device connected to a network is given a unique identifier, known as the IP address of the device. An IP address consists of four numbers separated by dots, for exmaple:
When network devices send messages to each other, they do so on a particular port, which can be thought of as a particular communication "channel". By using different ports, several services (e.g. web, email, file transfer) can operate independently on the same device.
By convention, port 80 is used to send and receive web pages.
Each local network operates on a particular subnet, which defines the scope of the network; it is only possible for a device to directly communicate with other devices that are on the same subnet.
The subnet is most commonly indicated by the first three numbers of the IP address, leaving the fourth number to indicate the devices themselves. The following IP addresses are all on the same subnet:
This arrangement is described by the subnet mask “255.255.255.0”, which means "use the first three numbers as the subnet, and use the fourth number as the device address".
Open the Network pane of System Preferences to see what addressing scheme is used by your local network.
DHCP is a technology, built into most routers, that automatically assigns IP address to network devices. This makes network setup very easy, however it does mean that the IP addresses of the devices on the network may change from time to time. This is unsuitable for any device that acts as a server (i.e. a network camera, or a Mac you wish to set up to stream video for remote monitoring), because in these cases the server must have a fixed address so that clients know where to connect to it.
On the Mac, this can be achieved using the "DHCP with manual address" setting in the Network pane of the System Preferences. Network cameras may also have such a setting, but if not they should be set up entirely manually with a unique IP address, and a subnet mask of 255.255.255.0).
Note that any manual address you assign for a network device should be outside the range of addresses that the router automatically assigns via DHCP, so as not to conflict. Unfortunately, routers vary in what ranges they use, but if you assign manual addresses in the range 200-250, this is normally quite safe. It is a good idea to look for this setting in your router so that you know which addresses are safe to use for your manual assignments.