Address allocation rules on S series switch

When an S series switch (except S1700 switches) functions as a DHCP server, the switch adds IP addresses in an address pool to different IP address lists based on the IP address status. Unallocated IP addresses belong to the allocatable IP address list. Allocated IP addresses belong to the in-use IP address list. Conflicting IP addresses belong to the conflicting IP address list. IP addresses that cannot be allocated belong to the unallocatable IP address list. The DHCP server selects an IP address for a client from the address pool in the following sequence:
1. IP address statically bound to the MAC address of the client on the DHCP server
2. IP address that has once been allocated to the client
3. IP address specified in the Option 50 field (requested IP address) in the DHCP Discover message sent by the client
4. Largest allocatable IP address
5. If the DHCP server does not find any allocatable IP address, it searches for the expired IP addresses and conflicting IP addresses in turn, and then allocates a valid IP address to the client. If all the IP addresses are in use, the DHCP server replies with a DHCP NAK message to notify the client that no IP address is available. After receiving the DHCP NAK message, the DHCP client sends a DHCP Discover message to apply for a new IP address.

The switch can exclude some IP addresses that cannot be allocated through DHCP from the address pool. For example, if 192.168.1.100/24 has been manually configured for the DNS server, the DHCP server needs to exclude IP address 192.168.1.100 from the address pool on network segment 192.168.1.0/24. In this way, IP address 192.168.1.100 will not be allocated through DHCP, preventing IP address conflicts.

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