CTI 2572-B / 2500C-2572-B Installation and Operation Manual 104
Subnet Mask
The designation of network classes proved to be very inflexible. For example, a Class A network assigned a
large number of host devices to the same IP network; potentially reducing performance, limiting topology,
and compromising network security. An additional entity, the Subnet Mask, was introduced to allow
segmenting an IP network into a collection of smaller networks called subnets.
The Subnet Mask is a collection of 32 bits that distinguish the network ID portion of the IP address from the
host ID. Subnet mask bits that belong to the network ID are set to 1. Subnet Mask bits that belong to the host
ID are set to 0.
To represent the mask, the 32-bit value is converted to dotted decimal notation or to CIDR (Classless Inter-
Domain Routing) notation. The CIDR notation is a modern alternative which counts the number of bits in
Network ID portion of the address (bits that are set to 1). The count is preceded by a slash. See the example
below.
NOTE
The binary representation of a Network Mask must be a single continuous block 1’s followed by a
contiguous block of zeroes. When entering the Network Mask in dotted decimal notation, you must
ensure that this requirement is maintained. For example, a network mask of 255.247.0.0 is not valid
because the binary equivalent (11111111111101110000000000000000) violates this rule.
The Network Mask must allow at least two bits of host address. In addition, a network mask which causes the
derived host ID to be 0 or a broadcast address (all Host ID bits set to 1) should not be used.
Using the Subnet Mask
The transmitting IP Host uses the Subnet Mask to determine where to send an IP message that has a unicast IP
address. After deriving the Network ID and Host ID portion of the IP Address using the Subnet Mask, the IP
Host compares the Network ID of the destination IP address with the Network ID of the Host IP address. If the
Network IDs are the same, the message is sent to another Host on the local network. If the Network IDs are
different, the message is sent to an IP Gateway, for routing to another network, if possible.
When you are configuring the IP address of devices that must communicate on the local network, you must
ensure that:
• The Subnet Mask of all devices are the same,
• The Network ID of all hosts are the same,
• The Host ID of each host is different.
If you are using subnet masks that are aligned with the IP address octets, this can easily be done by examining
the dotted decimal values. The octets of the IP address where the corresponding octet of the subnet mask is
255 belong to the Network ID and the octets of the IP address where the corresponding octet of the subnet
mask is 0 belong to the Host ID.
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