IP Helper Address

What is it and how to use it?

When the DHCP Server is placed on a different subnet from the all its clients. It is important that the router at the disparate subnets be configured with an “IP helper-address” specifying the Server as its pass-through broadcast node. Here is a sample command to enable such configuration:

CORE01 (config-if)#ip helper-address 192.168.0.1

Be advised that the “IP-Helper” feature opens up a suite of ports:

Protocol

UDP Port

Timeserver (not same as NTP port UDP 123)

37

TACACS

49

DNS

53

BOOTP/DHCP Server

67

BOOTP/DHCP Client

68

TFTP

69

NetBIOS name   service

137

NetBIOS datagram   service

138

IEN-116 name   service

42

To enhance the network security posture, it may be necessary to close any unused port with commands such as this (Cisco):

CORE01 (config-if)#no ip forward-protocol udp 49

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