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Id.-Nr.: 1556.212 .1-07
Appendix: Addressing in the Ethernet (basics)
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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 between the logic IP ad-
dress 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 address 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 request.
Ethernet address (MAC-ID)
MAC-ID (Media Access Control) is the fixed address that clearly identifies an Ether-
net 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 address: 192.168.181.1
Both a network and an individual participant are allocated an IP address in the net-
work. The IP address contains:
•
The Net ID (specifies a network address) and
•
the Host ID (specifies the address of an individual participant in the network). It
must be unique, i.e. no two terminal devices can be operated with the same Host
ID in the network.
so-called net mask (subnet mask) is used to determine which numbers of an IP ad-
dress 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 determines 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
Example:
In a small network with the net mask 255.255.255.0, at the IP address
192.168.181.1 the Net ID is 192.168.181 and the Host ID is 1. If (in a different,
medium-sized network) net mask 255.255.0.0 is set, then at IP address
192.169.100.1, the Net ID is 192.169 and the Host ID is 100.1.
Gateway
Networks with different Net IDs are connected together via routers or gateways. If a
network participant is to send data to a participant in a different network, the IP ad-
dress of the gateway must be additionally specified. For addressing the IP three de-
tails must be specified:
•
IP address
•
IP net mask
•
IP address of the gateway
Information on the addresses of your in-house network is available from the network
administrator.
Setting the IP address
The settings for IP addressing can be manually configured for each terminal unit. In
large networks this is done centrally and automatically by means of DHCP (Dynamic
Host Configuration Protocol). Here a DHCP server administers the IP addresses and
allocates them to the DHCP-capable terminal units. The LTI controls are DHCP-ca-
pable. For this, the
Enable DHCP
command must be active in the
Control configu-
ration
in MotionCenter. The IP address is then allocated by the DHCP server in the
network. During the boot-up the control then requests its IP address from the DHCP
server via the network
