3com 2928 User Guide

 

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Manual allocation: The network administrator assigns an IP address to a client like a WWW server, 
and DHCP conveys the assigned address to the client. 

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Automatic allocation: DHCP assigns a permanent IP address to a client. 

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Dynamic allocation: DHCP assigns an IP address to a client for a limited period of time, which is 
called a lease. Most DHCP clients obtain their addresses in this way. 

Dynamic IP address allocation process 

 

 
As shown in 

, a DHCP client obtains an IP address from a DHCP server via four steps: 

1)  The client broadcasts a DHCP-DISCOVER message to locate a DHCP server. 
2)  A DHCP server offers configuration parameters such as an IP address to the client in a 

DHCP-OFFER message. The sending mode of the DHCP-OFFER is determined by the flag field in 
the DHCP-DISCOVER message. 

3)  If several DHCP servers send offers to the client, the client accepts the first received offer, and 

broadcasts it in a DHCP-REQUEST message to request the IP address formally. 

4)  All DHCP servers receive the DHCP-REQUEST message, but only the server from which the client 

accepts the offered IP address returns a DHCP-ACK message to the client, confirming that the IP 
address has been allocated to the client, or a DHCP-NAK unicast message, denying the IP 
address allocation. 

 

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After the client receives the DHCP-ACK message, it will probe whether the IP address assigned by 
the server is in use by broadcasting a gratuitous ARP packet. If the client receives no response 
within the specified time, the client can use this IP address. Otherwise, the client sends a 
DHCP-DECLINE message to the server and requests an IP address again. 

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IP addresses offered by other DHCP servers are still assignable to other clients.