Shared network declarations in DHCP are used to group together different logical subnets that share the same physical network.
For example, consider a network with 250 workstations on a physical network with the logical address ID of 192.168.6.0/24. You need to add 100 workstations to this physical network, but the only available subnet ID is 192.168.12.0/24. If the subnets were contiguous (that is, 192.168.6.0/24 and 192.168.7.0/ 24), you could modify the subnet mask to create a single logical subnet to accommodate the additional computers (192.168.6.0/23). However, the two network IDs are not contiguous.
By configuring a shared network, you can group the two networks together. The benefit is that your DHCP server can allocate IP addresses from the common shared network to any host on either of the networks, without the need to isolate the networks to different router interfaces.
Tag groups and tags are the mechanism by which subnets are grouped into DHCP shared networks. To use shared networks, you need to associate a single tag group with a configuration. A configuration can have many associated tags, but only one tag that is associated for the purpose of forming shared networks.