Assigning Addresses With The Dhcp Client - Siemens SCALANCE X-300 Configuration Manual

Assignment of an IP address

3.5 Assigning addresses with the DHCP client

3.5 Assigning addresses with the DHCP client
Properties of DHCP
DHCP (Dynamic Host Configuration Protocol) is an expansion of BOOTP; however, there
are several important differences compared with BOOTP:
● The use of DHCP is not restricted to the boot phase; DHCP can also be used during
normal operation.
● The assigned IP address remains valid only for a particular time known as the lease time.
Once this period has elapsed, the client must either request a new IP address or extend
the lease time of the existing IP address.
● There is normally no fixed address assignment; in other words, when a client requests an
IP address again, it normally receives a different address from the previous address. It is,
however possible, to configure the DHCP server so that it assigns a fixed address.
Note
As soon as the IP address has been assigned once by a PROFINET IO controller, DHCP
automatically deactivates itself and must be reactivated if required.
Note
DHCP uses a mechanism with which the IP address is assigned for only a short time (lease
time). If the IE switch does not reach the DHCP server for a new request on expiry of the
lease time, the assigned IP address, the subnet mask and the gateway are changed to static
entries.
The device therefore remains accessible under the last assigned IP address even without a
DHCP server. This is not the standard behavior of office devices but is necessary for
problem-free operation of the plant.
Since the DHCP client also sends a RELEASE to the server, the server can assign this
address to a further device so that inconsistencies can occur within the network.
Remedy:
After disabling DHCP, you should therefore either
• change the IP address of the IE switch to an address not assigned by DHCP or
• remove the IP address assigned to the device from the address pool of the DHCP server.
Working with a mixture of dynamic address assignment and statically assigned addresses is
not advisable.
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Configuration Manual, 10/2014, C79000-G8976-C187-22
SCALANCE X-300 / X-400