3Com 8807 Configuration Manual page 682

682 C 57: DHCP C
HAPTER ONFIGURATION
Automatic IP address assignment. The DHCP server automatically assigns fixed
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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.
Dynamic IP address assignment. The DHCP server leases IP addresses to DHCP
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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.
2 IP address assignment order.
The DHCP server assigns IP addresses except the forbidden ones to clients in the
following orders.
IP addresses in the address pool of the DHCP server that are statically bound to
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the MAC addresses of the DHCP clients.
IP addresses that are reclaimed by the DHCP server. That is, those in the
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Requested IP Addr Option fields of DHCP Discover packets sent by DHCP
clients.
The first available IP address in the address pool the DHCP server finds.
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The first expired or once conflicted IP address it finds. A DHCP server returns an
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error if it cannot find any available IP address from all these types of IP
addresses when assigning an IP address.
3 Types of address pools of DHCP server
Global address pool, valid for the entire switch. An address pool of this type is
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created using the dhcp server ip-pool command in system view.
VLAN interface address pool, valid for a specific VLAN interface. An address
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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.
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:
Discovery. The DHCP client tries to find a DHCP server by broadcasting a
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DHCP_Discover packet in the network. (Only DHCP servers respond to this type
of packet.)
Provision. Each DHCP server that receives the DHCP_Discover packet selects an
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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.
Selection. The DHCP client only receives the first arriving DHCP_Offer packet if
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there are DHCP_Offer packets from several DHCP servers. Then, it retrieves the