Startup and functions
7190_en_02
PHOENIX CONTACT
3-19
Possible address combinations
Figure 3-20
Structure of IP addresses
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.
7 Bit
24 Bit
14 Bit
16 Bit
21 Bit
8 Bit
28 Bit
27 Bit
Netz-ID
Host-ID
Host-ID
Host-ID
Netz-ID
Netz-ID
Identifikator für Multicast-Gruppe
Reserviert für zukünftige Anwendungen
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
Klasse A
0.0.0.0 - 127.255.255.255
Klasse B
128.0.0.0 - 191.255.255.255
Klasse C
192.0.0.0 - 223.255.255.255
Klasse D
224.0.0.0 - 239.255.255.255
Klasse E
240.0.0.0 - 247.255.255.255
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