routable ip

That's right. And the non-routable addresses are non-routable because that was the design decision taken when the internet was "invented", see RFC1818: www.rfc-editor.org/rfc/rfc1918.txt . Exactly how the blocking is done I'm not sure - but I'm certain someone out there will shortly tell us...

that's false, the ips mentioned there ARE routable, but they are NOT globally routable (they can not leave your network), www.rfc-editor.org/rfc/rfc3330.txt documents the special ip ranges, some of which are non-routable.

as for the blocking of non globally routable ips (private ones), i usually do it at firewall level to make sure it doesn't confuse my firewall. not sure if there is some other mechanism implemented that prevents the private ips from leaving the network.

Hi redduck666, if you read Bishops previous post he does state that its non-routable over the Internet.

It is quite common practice to block private addresses on firewalls in order to help guard against spoofing/DoS attacks, when its configured on the egress. This is known as RFC 1918 filtering.

It kind of comes back to what I was saying; All IP addresses are routable, but not all are permitted to be routed over the internet. And in actual fact the non-routable IP addresses CAN leave your network and be sent out onto the internet - they just won't go anywhere when they do because the internet routers will drop them

It kind of comes back to what I was saying; All IP addresses are routable

even 127/8?

even 127/8?

Theoretically yes. However you cannot configure IP addresses in this range on Cisco devices.
[code:1]R1(config)#interface Loopback 0
R1(config-if)#ip address 127.1.1.1 255.255.255.255
Not a valid host address - 127.1.1.1[/code:1]
So I think we may say 127/8 is not routable