Private network IPv4 ranges are:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
Since the IP address ranges you're receiving are from your ISP (you can look them up on a site like arin.net) you're receiving DHCP passthru addresses because your Airport Extreme is in Bridge mode (page 31).
You can better protect your local network by disabling bridge mode, since bridge mode is meant to be used in situations where you're using your own DHCP server to give out addresses and not use the one built into the router. It makes the router act like an access point instead of a router.
This will not slow down your internet connection, but you will no longer be able to connect to your devices directly from the internet without going through the router. Then you can connect to devices on your local network from within your local network, and you'll have better control over the IP addresses you're receiving since you'll be assigning DHCP addresses from the Airport Extreme itself.
They label it in the interface as "connection sharing" which unfortunately confuses people. Also they do mention that you'll get different IP addresses, but they're talking about the IP addresses on the internet (external IPs) as opposed to local LAN addresses (which you will have when you enable DHCP outside of bridged mode).