Sure!
IP addresses are assigned to every device on the network including routers and network switches.
A subnet is a network which has one more devices. The modern notation used for IPv4 networks is called CIDR (instead of using Class A, B, or C networks). For home networks it is common to use the 192.168.x.x addresses so a subnet could be defined as 192.168.0.0/24 or 192.168.1.0/24, etc. The /24 indicates that the last digit can be used for device addresses so for 192.168.0.0/24, you can have devices assigned to 192.168.0.1-192.168.0.254 (.0 and .255 are reserved for special use such as network broadcasts).
In OPNsense, you will see 2 gateways by default if IPv6 is enabled. One gateway on the WAN is used for IPv4 and the other is IPv6. IPv6 is the new protocol for IP addresses that allows for a much greater amount of IP addresses than IPv4. It is ok to have both enabled. Sometimes devices/software will prefer to use the newer IPv6 protocol if it's enabled. You have to keep that in mind when creating firewall rules. If you want to restrict the traffic for both IPv4 and IPv6 network traffic, you need to apply the rules to both protocols.
If you wish to see the names of the devices, you need to set the option to "register DHCP leases" and "register DHCP static mappings" on the "Services > Unbound DNS > General" page. This doesn't always guarantee you will see the hostname. I've had some devices not show up but most do. If you really want everything named better, you could create a static DHCP mapping for a device (once you have identified it) and you can set an IP address (outside of your DHCP IP address range you have set for the network) and a hostname. Sometimes the manufacturer will show up below the MAC address which could possibly help identify devices. Most devices provide a way for you to view the IP address (and sometimes the MAC address). That will help you find out which device has which IP address.
Please let me know if this info helps and if you have more questions!
