Jails - IP Addresses

Each time a jail is started, FreeBSD's jail(8) utility will tell ifconfig to create a simple alias to your jail's designated ip address. However that only occurs on the FreeBSD host machine. In most situations, your local network is under the control of a local router. So to prevent ip address conflicts elsewhere you should reserve your jail's ip address on your local router too.

Reservation Strategies

1) Use High-IPs

Most router DHCP servers usually allocate ips in ascending order. So one strategy is simply give jails a high-enough ip addresses that they will not be likely to cause conflicts.

For example: start your jails at "192.168.1.100", or "192.168.1.200" etc.
Not the most flexible / efficient way to allocate IP addresses. There may be some wastage.
Be aware that the highest IP address is going to be about "192.168.1.255"

2) Alter the DHCP range

Optionally, you may also decide restrict your home router's DHCP allocation range. The requires going into your router's DHCP settings and changing the first / last IP address.

A DHCP range of "192.168.1.2" --> "192.168.1.100" will not conflict with jails that start at ".101".
Or you could make the DHCP start at a higher IP, leaving the lower IP range available for jails.
Still not the most flexible / efficient way to allocate IP addresses. There may be some wastage.

3) Static DHCP - Multi-IP

This option depends entirely on your router's capabilities. Unfortunately for most home routers it's not possible to assign multiple static IPs to the a single mac address. Normally only 1 IP can be assigned to each MAC address and no more. However if your specific router supports it, then by all means use this method.

With static DHCP you can assign any arbitrary IP address within the same mixed DHCP range.
The same DHCP server is then aware of your jail's IP address and won't assign it to any other devices.
Feature usually not capable of assigning multiple IPs to only 1 MAC address.

4) Static DHCP - fake MAC address

If your router does not support method 3), you may still be able to use Static DHCP. Just create your "static DHCP" entries with FAKE mac addresses.