Private and Public IP Address Ranges

Marco Cardazzi

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On October 17th In IP Solutions, IP Tech Tips, Networking, Video Surveillance
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IP enabled devices each have a “private” or “local” IP address that identifies that device on the local network to which it’s connected. Each of these addresses must be unique on that network, with no replication of a single address.

Networks that are connected to the Internet will have a “public” or “Internet” IP address that is provided by the Internet Service Provider (“ISP”). This address identifies the specific LAN with a unique address on the Internet.

The Internet Assigned Numbers Authority (IANA) established certain blocks of IPV4 addresses for the use of private (LAN) and public (Internet) addresses. The private IP addresses will fall within these specific ranges:

Most LAN networks use the “Class C” private IP address range. So in a typical network in a home or business the local IP addresses will start with 192.168. In a Class C network the first three groups of the IPV4 address must match exactly for the devices to be able to communicate. So it is common for local network devices to have an addressing scheme such as 192.168.1.1-254, with the first three octets matching and the last providing a unique address for each device.

What defines the different classes of IP addresses is the “subnet mask” which is associated with that class. Normally the subnet mask is the same in every device on a single LAN. If we look at the Class B private addresses above, we see that the default subnet mask is 255.255.0.0. The first two octets of this subnet mask with the “255s” indicate that those sections of the IPV4 address must match on all devices that need to communicate with each other. So in a Class C network the default subnet mask says that the first three octets must match exactly.

Public IP addresses will be issued by an Internet Service Provider and will have number ranges from 1 to 191 in the first octet, with the exception of the private address ranges that start at 10.0.0 for Class A private networks and 172.16.0 for the Class B private addresses.

It’s important for technicians to understand that IT personnel can choose to use any of the private address ranges for their LAN devices. It is not at all uncommon for a technician to be confronted with a client’s network where the local addresses are in the range of 10.0.0.1-254, and the subnet mask used is 255.255.255.0. This is an example of using Class A private addresses with a Class C subnet, which makes this a Class C network. It is the subnet mask that defines which “class” a LAN network’s addressing is using.

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