Product designation
3-6
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6874_en_16
3.2.2.3
Special IP addresses for special applications
Certain IP addresses are reserved for special functions. The following addresses should not
be used as standard IP addresses.
127.x.x.x addresses
The Class A network address "127" is reserved for a loopback function on all computers,
regardless of the network class. This loopback function may only be used on networked
computers for internal test purposes.
If a telegram is addressed to a computer with the value 127 in the first byte, the receiver
immediately sends the telegram back to the transmitter.
Correct installation and configuration of the TCP/IP software, for example, can be checked
in this way.
As Layers 1 and 2 of the ISO/OSI reference model are not included in the test they should
be tested separately using the ping function.
Value 255 in the byte
Value 255 is defined as a broadcast address. The telegram is sent to all the computers that
are in the same part of the network. Examples: 004.255.255.255, 198.2.7.255 or
255.255.255.255 (all the computers in all the networks). If the network is divided into
subnetworks, the subnet masks must be observed during calculation, otherwise some
devices may be omitted. Simplified: The last address of an area is reserved as the
broadcast address.
0.x.x.x addresses
Value 0 is the ID of the specific network. If the IP address starts with a zero, the receiver is
in the same network. Example: 0.2.1.1 refers to device 2.1.1 in this network.
The zero previously signified the broadcast address. If older devices are used,
unauthorized broadcast and complete overload of the entire network (broadcast storm) may
occur when using the IP address 0.x.x.x.
3.2.2.4
Subnet masks
Routers and gateways divide large networks into several subnetworks. The IP addresses
for individual devices are assigned to specific subnetworks by the subnet mask. The
network part
of an IP address is
not
modified by the subnet mask. An extended IP address
is generated from the user address and subnet mask. Because the masked subnetwork is
only recognized by the local computers, this extended IP address appears as a standard IP
address to all the other devices.
Structure of the subnet mask
The subnet mask always contains the same number of bits as an IP address. The subnet
mask has the same number of bits (in the same position) set to "one", which is reflected in
the IP address for the network class.
Example: An IP address from Class A contains a 1-byte network address and a 3-byte
computer address. Therefore, the first byte of the subnet mask may only contain "ones".
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