DHCP > DHCP Pool
DHCP Pool
A DHCP Pool is the set of IP address pools that will be assigned to the DHCP clients. You can add several address pools in a DHCP pool and each DHCP pool means the VLAN(Virtual local area networks).
If a DHCP server receives a DHCP request, the server assigns an IP address based on the subnet from which the message was received. If the client is on the same subnet as the server, the server selects an IP address from the DHCP pool that has the same subnet IP pool as the server's IP address and assigns it to the client. If the client is not on the same subnet as the server, the server selects an IP address from the DHCP pool that has the same subnet IP pool as the relay agent's IP address and assigns it to the client.
If a VLAN has multiple IP subnets, the IP address of the relay agent may the primary VLAN address. Therefore, a DHCP pool must contain the IP pool for the primary subnet.
The imRAD DHCP server can assign IP addresses randomly or sequentially and you can configure it at the DHCP > Settings > General.
The rfc 7721 recommends the random assignment. Thus, these addresses can be considered to be "stable, semantically opaque".[1]
New DHCP Pool
You can create a new DHCP pool and an IP address pool by click the "Create New DHCP pool" button at the right top of the DHCP pool list.
| Items | DHCP Version | Required | Description |
|---|---|---|---|
| DHCP Version | Yes | Choose the DHCP version. | |
| Name | 4/6 | Yes | It is the pool name to distinguish from others. |
| Lease Time | 4 | Yes | It is the DHCPv4 Lease Time. |
| Preferred Time | 6 | Yes | It is the Preferred Lifetime . |
| Valid Time | 6 | Yes | It is the Valid Lifetime . |
| Preferences | 4/6 | No | Auto Reservation: If you enable it, an IP address that was assigned to a host is automatically reserved for the host. |
| 6 | No | Stateful DHCPv6 service: You must enable the Stateful DHCPv6 service to assign IP addresses to clients. Please refer to the Stateful vs stateless DHCPv6 for more. | |
| Status | 4/6 | Yes | "Disable" means that the pool is created but not service from it. |
| Identity Association | 6 | Yes | Choose one of them. |
| Default Gateway / Subnet | 4 | Yes | Input the IPv4 gateway address and the bit number of subnet between 1 and 30. |
| Prefix | 6 | Yes | Input the prefix of the IPv6 address. |
| Start IP | 4/6 | Yes | Input the first IP address to be assigned. |
| Range Count | 4/6 | Yes | Input the total number of IP addresses in the pool. |
Additional IP Address Pool
If a VLAN is configured with several secondary VLANs, you must add them all into a DHCP pool.
If you click the last icon(
) on a DHCP pool, it shows a dropdown menu.
And then, select the "Add IP Address pool" menu to add more IP address pools in a DHCP pool.
If a DHCP pool has an IPv4 address pool, you can not add an IPv6 address pool but only an IPv4 address pool and vice versa.
IP Address Pool List
You can view all the DHCP pools and their summary information, and if you click the name of a DHCP pool, It shows and hides all IP address pools in the DHCP pool. The following table shows the description of several items on the list.
| Item | Description | |
|---|---|---|
| Lease Time | It indicates the lease time of a DHCPv4 pool or the preferred time and valid time of a DHCPv6 pool.
If the preferred time is the same as the valid time, It only shows the preferred time. | |
| Preferences | If the Auto Reservation or the Stateful DHCPv6 is set, display each. | |
| Range Count | IP Address Pool | It is the total number of IP Address Pools in a DHCP pool. |
| Available | It indicates that the total number of available IP addresses in a DHCP pool.
The total number of IP addresses minus the "Allocated" equals the "Available". | |
| Allocated | It is the total number of allocated IP addresses. | |
| Reservation | It is the total number of reserved IP addresses. | |
| Exclusion | It is the total number of excluded IP addresses. | |
If you click the first icon of on a DHCP pool, it shows the assignment of IP addresses in a DHCP pool in real-time.
If you want to change the values(i.e. Name, the Lease time, Preferences, Failover) of a DHCP pool, click the second icon of .
If you want to change the Status, Default Gateway, Start IP, or Range count of an IP Address Pool, click the icon(i.e. 
) on the right of each IP Address Pool.
IP Address List
It shows all IP addresses that are in use or were in use by click the name or the 
icon on an IP Address Pool.
For more information about the IP Address list, Please refer to the IP Address list section.
DHCP Reservation
The DHCP Reservation enables hosts can be assigned the same IP address every time in the same subnet. in a DHCPv4, the reservation consists of an IPv4 address and the MAC Address of a host. in a DHCPv6, the reservation should contain an IPv6 address, DUID, and IAID of a host.
Auto Reservation
As mentioned in the preference of the "New DHCP Pool" table, if you enable the "Auto Reservation" of a DHCP Pool, an IP address of the DHCP Pool is reserved automatically after assigning it to a host. If IP addresses that were automatically reserved are not using more than a specific day, the addresses will be deleting. The specific day is " Dormant DHCP Reservation"
"Auto Reservation" is configured in each DHCP pool.
UserName Reservation
The UserName Reservation enables a host to be assigned an IP address based on the User-Name of the RADIUS attribute. A host is assigned a reserved IP address for the User-Name that was authenticated and authorized through the imRAD RADIUS. If a NAS does not send the Accounting, It can't accomplish the User-Name Reservation.
If you want to reserve an IP address, click the last icon() on a DHCP pool. And then, select the "Add DHCP Reservation" on the dropdown menu. If a DHCP pool is for the IPv4, you can only reserve an IPv4 address.
| Item | Version | Required | Description |
|---|---|---|---|
| DHCP Pool | 4/6 | Yes | It is the DHCP Pool that will contain the IP address to be reserved. |
| IP Address | 4/6 | Yes | It is the IP address to be reserved. |
| User-Name Reserv. | 4/6 | No | To reserve an IP address based on the RADIUS User-Name, turn on this. |
| MAC Address | 4 | Yes, No | It shows this field only when you reserve an IPv4 Address. You must input the MAC Address of a host that will be assigned the IPv4 Address.
If you turn on the User-Name Reserv, you don't need to input this. |
| DUID, IAID | 6 | Yes, No | It shows this field only when you reserve an IPv6 Address. You must input the DUID and IAID of a host that will be assigned the IPv6 Address.
If you turn on the User-Name Reserv, you don't need to input this. |
| RADIUS Username | 4/6 | Yes, No | If you turn on the User-Name Reserv, you must input the User-Name to be authenticated through the imRAD RADIUS. |
| Description | 4/6 | No | It is the description of the reservation. |
If an IP address is automatically reserved by the "Auto Reservation" preference in a DHCP pool, the description of a reservation is written as "AUTO Reservation".
If you click the "Reservation" tab after expanding the DHCP pool that includes the reserved IP addresses, It shows all the reserved. You can edit or delete a reserved IP address at the list.
DHCP Exclusion
The DHCP Exclusion defines the range of IP address that should not be assigned. Some network devices need to use statically assigned IP addresses. If such IP addresses are in a DHCP pool, you must exclude them from the pool.
If you want to add an exclusion range, click the last icon() on a DHCP pool. And then, select the "Add DHCP Exclusion" on the dropdown menu. The exclusion range should be inside of the IP Address pools.
| Item | Required | Description |
|---|---|---|
| DHCP Pool | Yes | It is the DHCP Pool that will contain the exclusion range. |
| Start IP | Yes | It is the first IP address to be excluded. |
| Last IP | Yes | Input the number of IP address to be excluded from the start IP address. |
| Description | No | It is the description of the exclusion. |
DHCP Server Options
You can specify DHCP Server options globally or exclusively for a specific DHCP pool.
If you select the "Add DHCP Server Option" after click the last icon() on a DHCP pool, you can add a DHCP server option.
The DHCP Server option configuration section explains the details of the DHCP Server Options.