User Documentation

Appendix: Addressing in the Ethernet (basics)
System manual
2696790000/02/04.2020
120
20 Appendix: Addressing in the Ethernet (basics)
Due to the separation of logic and physical protocol levels (Ethernet and
TCP/ IP) two types of addresses exist in the network:
● A fixed Ethernet address (MAC-ID) for each unit and
● an IP address, which is allocated to each unit in the network.
The application always sends data to or receives data from an IP address.
To ensure their arrival at the receiver a connection must be established be-
tween the logic IP address and the physical Ethernet address. The Address
Resolution Protocol ARP serves this purpose. An ARP table is stored in
each network PC, which specifies the corresponding physical Ethernet ad-
dress for the IP addresses of the network. If the ARP table does not list an
Ethernet address, the IP driver can generally determine it via an ARP re-
quest.
Ethernet address (MAC-ID)
MAC-ID (Media Access Control) is the fixed address that clearly identifies an
Ethernet device.
IP address
An IP address according to the standard IPv4 is generally specified with 4
decimal numbers divided by points (each 1 byte). Example for an IP ad-
dress: 192.168.181.1
Both a network and an individual participant are allocated an IP address in
the network. The IP address contains:
● The Net ID (specifies a network address) and
● the Host ID (specifies the address of an individual participant in the net-
work). It must be unique, i.e. no two terminal devices can be operated
with the same Host ID in the network.
A so-called net mask (subnet mask) is used to determine which numbers of
an IP address represent the Net ID and the Host ID.
Net classes
With a "0" as wildcard the net mask for IP addresses defines which bits are
used for addressing the participant (Host ID). A "1" as wildcard defines which
bits the network address (Net ID) contains. The number of these bits deter-
mines which classes the networks belong to:
Net class Net mask Description
A 255.0.0.0 Large network
B 255.255.0.0 Medium-sized network
C 255.255.255.0
Small network with a maxi-
mum of 254 participants