The concept of routing packets based on their destination IP address range is central to the
design of the Internet and has evolved over time resulting in several different terms and
notations being used.
In practical terms, each device on an Internet Protocol (IP) network must be assigned a unique
IP address from the range of IP addresses designated for that network. Since globally unique IP
addresses are in short supply, most small office or home LANs use addresses from a range
designated for "private use" (192.168.x.x or 10.x.x.x). An Internet Access Router is then used
to translate these addresses to a single public IP address supplied by an Internet Service Provider
A "subnet mask" or "prefix length" is just a convenient shorthand for specifying a range of IP
addresses. An IP address can be represented as a 32-bit binary number. To define a range of
addresses, we divide these 32-bits into a network part and a host part. The subnet mask or
prefix length tells us where the split occurs. If our router is assigned a local address of
192.168.0.1 and we want to indicate that this is from a range of 256 addresses, we can write
this as 192.168.0.1/24 where the "/24" is the prefix length and indicates that the first 24-bits
are the network number and the last 8-bits are the host number. Alternatively, we can
specify a subnet mask of 255.255.255.0 which effectively says the same thing.
Choosing An IP Address for your Whozz Calling? Unit(s)
The first step is to determine the range of valid IP addresses for your LAN (Local Area Network).
You can find this in the Network Preferences Panel, or by running Apple's Network Utility and
clicking on the Interface Info tab.
By convention, the first address (host number 1) is usually reserved for your Internet Access
Router, and the last address (host number 255) is reserved for broadcasting to all the hosts on
your LAN (the subnet broadcast address).
If computers on your LAN are normally configured using DHCP (assigned addresses by your
router), the configuration software for your router will have a place to specify the starting and
ending address, or range of addresses that can be dynamically assigned. You will want to choose
an IP address outside of this range for each Whozz Calling? unit on your LAN.
If you have an AirPort Base Station for example, launch the "AirPort Utility", press "Manual
Setup", and navigate to "Internet -> DHCP". I use a "DHCP Beginning Address" of .8 to leave a
few fixed addresses for other devices. In the "DHCP Reservations" section below I then list the
fixed IP addresses I have assigned along with their descriptions.
To reserve a DHCP Address, you'll need to know the Ethernet MAC address (or DHCP ClientID) of
the corresponding device. A convenient way to find the MAC address of devices on your LAN is
to use the Address Scan tool in IPNetMonitorX. Notice you can reserve a DHCP address outside
the dynamically assigned address range. In the example above, the LaserJet 4600 and
SPA-2102 can still be configured via DHCP. When they contact the DHCP server, they will always
be assigned the same reserved address so other hosts on your LAN will know where they are
(without relying on service discovery).
More On Terminology
A range of IP addresses defined by a subnet mask is sometimes called an "IP subnet". Some
references use the term "network mask" in place of subnet mask. Historically, subnet masks
were not required to be contiguous (all 1's followed by all 0's), but this practice is discouraged
and no longer widely used because it was less efficient. As the Internet expanded, the need to
keep routing and address assignment as efficient as possible became very important. Specifying
network ranges as an IP address and prefix length is the preferred technique adopted by the
next generation of Internet Protocols, and is sometimes referred to as "CIDR notation" (Classless