Exclude IP address from DHCP pool

I'm going to preface this answer (that I already typed out, and am thus, not going to delete) by saying that it looks like your router uses the term "static DHCP" for what the rest of the world calls a "DHCP reservation."

So, in fact, the proper configuration of your device is to have your DHCP pool set from 192.168.0.2 to 192.168.0.254, and then to put all those static addresses I see in the LAN hosts table into the "Static DHCP" client table. (If you don't put them in the Static DHCP table, the DHCP server may assign those IP addresses to another device, creating an IP conflict.)

Also...

1. I have almost no experience using Cisco Small Business gear, so I'm not 100% sure of the above, but that's sure what it looks like.
2. Your use of the subnet designation (/24) is not correct, and confusing.
   • If you're punching that into your config, it's going to cause problems. If you're just typing it that way, that's better... but still wrong and confusing.
   • 192.168.0.241/24 is not a valid subnet. It's 192.168.0.0/24. The /24 indicates that it is a 24 bit subnet composed of 256 IP addresses.

Anyway, moving onto the general case...

Shouldn't we exclude a static IP from the DHCP pool?

In short, yes. You should exclude any statically-assigned IP addresses from being assigned from your DHCP pool. If you don't, your DHCP server will happily assign that static IP address to a device requesting a DHCP-assigned address, and you'll end up with an IP conflict, and some potentially confusing connection issues reported by your users.

As to how you typically accomplish this, there are basically three ways.

1. Actually exclude the IP address from the DHCP pool range.
   • This will make the DHCP server ignore that IP address, as if it doesn't exist.
     
     
2. Create a DHCP "reservation."
   • This will force the DHCP server to always assign that "reserved" IP address to the device you specify (or any device with the same MAC address, to be precise).
   • It looks like your router calls that option a "static DHCP." (Since DHCP stands for "Dynamic Host Configuration Protocol," this is a dumb name.)
     
     
3. Use hostnames and DNS instead.
   • Instead of messing around with IP addresses that the majority of devices don't care about and users won't remember, assign a hostname, use a nameserver and access your devices by hostname instead of IP, so you can let everything get a dynamic address from your DHCP server and not care what the actual IP address is.

Given your interface setup you have to put BOTH subnets (10.0.106.0/255.255.254.0 & 192.168.120.0/255.255.255.0) in THE SAME shared-network declaration.

From dhcpd.conf man page:

The shared-network statement is used to inform the DHCP server that some IP subnets actually share the same physical network. Any subnets in a shared network should be declared within a shared-network statement. Parameters specified in the shared-network statement will be used when booting clients on those subnets unless parameters provided at the subnet or host level override them. If any subnet in a shared network has addresses available for dynamic allocation, those addresses are collected into a common pool for that shared network and assigned to clients as needed. There is no way to distinguish on which subnet of a shared network a client should boot.

You probably can get around the restriction in the last sentence by properly using allow, deny and host declarations and you seem to be going in right direction.

You may also want to have a look at allow/deny members of "class". I am not sure however if it is available on your version of the DHCP server. It is documented in man page for ISC dhcpd 4.2.5 on CentOS 7.