3Com DUA1750-2BAA01 II. Communications between DHCP clients and DHCP server

3Com Switch 8800 Configuration Guide Chapter 13 DHCP Configuration
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z Automatic IP address assignment. The DHCP server automatically assigns fixed
IP addresses to DHCP clients when they connect to the network for the first time.
After that, the IP addresses are always occupied by the DHCP clients.
z Dynamic IP address assignment. The DHCP server leases IP addresses to DHCP
clients for predetermined period of time and reclaims them at the expiration of the
period. In this case, a DHCP client must reapply for an IP address regularly. This is
the common case for normal users.
IP address assignment orderThe DHCP server assigns IP addresses except the
forbidden ones to clients in the following orders.
z IP addresses in the address pool of the DHCP server that are statically bound to
the MAC addresses of the DHCP clients.
z IP addresses that are reclaimed by the DHCP server. That is, those in the
Requested IP Addr Option fields of DHCP Discover packets sent by DHCP clients.
z The first available IP address in the address pool the DHCP server finds.
z The first expired or once conflicted IP address it finds. A DHCP server returns an
error if it cannot find any available IP address from all these types of IP addresses
when assigning an IP address.
2) Types of address pools of DHCP server
z Global address pool, valid for the entire switch. An address pool of this type is
created using the dhcp server ip-pool command in system view.
z VLAN interface address pool, valid for a specific VLAN interface. An address pool
of this type is created by the system when the VLAN interface is configured with a
legal unicast IP address and you specify to assign IP addresses in VLAN interface
address pool using the dhcp select interface command in VLAN interface view.
The address range of the available addresses is that of the network segment the
VLAN interface resides.
II. Communications between DHCP clients and DHCP server
To obtain valid dynamic IP addresses, the DHCP clients exchange different information
with the DHCP server in different phases. Usually, three modes are involved:
1) First round registration
A DHCP client goes through the following four steps when it accesses a network for the
first time:
z Discovery. The DHCP client tries to find a DHCP server by broadcasting a
DHCP_Discover packet in the network. (Only DHCP servers respond to this type
of packet.)
z Provision. Each DHCP server that receives the DHCP_Discover packet selects an
available IP address from an address pool and sends a DHCP_Offer packet that
carries the selected IP address and other configuration information to the DHCP
client.