Question about DHCP

[azhdar1234]

Which way is DHCP-server can share ip addresses to different subnet ? How DHCP is understand what this ip should forward to exactly this subnet ?

[devils_advocate]

Cisco provide a document which explains it fully.

http://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/27470-100.html

Effectively the default gateway of the local subnet relays the DHCP discover message to the DHCP server and also relays the response back to the client. The gateway replaces the Source IP of the DHCP Discover message with its own IP address so it can route it to the DHCP server and the server can route it back.

Fairly simple really. 

[Joseph W. Doherty]

Just to expand a bit, the "trick" (if I remember correctly) is the initial DHCP forwarding device includes the IP address of the gateway which received the DCHP discovery.  This allows the DHCP server to not only know what device to send the DHCP offer back to, but what subnet the host is on.