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Configuration Guide - IP Service

S2720, S5700, and S6720 V200R013C00

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DHCP policy VLAN, DNS, mDNS gateway, mDNS relay, UDP Helper, IP performance optimization, IPv6, DHCPv6, IPv6 DNS, IPv6 over IPv4 tunnel, and IPv4 over IPv6 tunnel.
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DHCP Leases and Address Pools

DHCP Leases and Address Pools

DHCP Leases

A lease is defined as the time period for which a DHCP server allocates an IP address to a client. The lease can be extended upon subsequent requests. If the client no longer needs the IP address, it can release the address back to the server before the lease expires. The server is then free to assign this address to a different client if no other idle IP address is available.

The lease period configured for a DHCP server applies to all of the IP addresses that a DHCP server dynamically assigns to its clients. A different DHCP server may have a different lease term for its clients. A statically allocated IP address is not subject to the lease terms.

According to RFC 2131, a DHCP client does not wait for its lease to expire, because it may be assigned a different IP address. Instead, when a DHCP client reaches the halfway point of its lease period, it attempts to extend its lease so that it retains the same IP address.

Address Pools

An address pool is a set of all the IP addresses that a DHCP server has reserved for dynamic client allocation. Along with each IP address, the server stores certain network parameters, such as a default lease length for the IP address and other configuration parameters (for example, a subnet mask and the address of the default gateway) to be sent to the client when it is assigned that IP address.

Address pools are classified into interface address pools and global address pools.

  • Interface address pool: After an IP address is configured for an interface on a DHCP server, you can create an address pool on the same network segment as this interface. Addresses in the address pool can be allocated only to clients connected to the interface. The interface address pool can allocate IP addresses to clients on the same network segment as the DHCP server.
  • Global address pool: On a DHCP server, you can create an address pool on the specified network segment in the system view. Addresses in the address pool can be allocated to all clients connected to the DHCP server, even if the server and clients are on different network segments (providing that a DHCP relay agent is used).

A DHCP server selects address pools according to whether a DHCP relay agent is deployed. When no relay agent is deployed, the server selects the address pool on the same network segment as the IP address of the interface receiving DHCP Request messages. When relay agents are deployed, the server selects the address pool on the same network segment as the IP address specified in the giaddr field of received DHCP Request messages.

The number of IP addresses required in an address pool depends mainly on the number of clients that will connect to the network and the frequency at which they connect and disconnect.

In an address pool, the status of an IP address determines to which IP address list it is added. Unallocated IP addresses are added to the allocatable IP address list, allocated IP addresses are added to the in-use IP address list, conflicting IP addresses are added to the conflicting IP address list, and IP addresses that cannot be allocated are added to the unallocatable IP address list.

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Updated: 2019-04-20

Document ID: EDOC1100065678

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