Why can't I ping an address on the loopback device under FreeBSD?

From Wikipedia:

The most commonly used IP address on the loopback device is 127.0.0.1 for IPv4, although any address in the range 127.0.0.0 to 127.255.255.255 is mapped to it.

This is not true, at least on FreeBSD:

$ ping 127.1.1.1
PING 127.1.1.1 (127.1.1.1): 56 data bytes
ping: sendto: Can't assign requested address

Is this correct behaviour?

FreeBSD (also OS X, and I believe NetBSD & OpenBSD) will respond to requests sent to configured addresses on the loopback interface, just as they would for addresses on any other interface -- If you want an answer you need to assign the address first:

mgraziano@monitor ~]$ ifconfig lo0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
    options=3<RXCSUM,TXCSUM>
    inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3 
    inet6 ::1 prefixlen 128 
    inet 127.0.0.1 netmask 0xff000000 
    nd6 options=3<PERFORMNUD,ACCEPT_RTADV>

[mgraziano@monitor ~]$ ping 127.1.1.1
PING 127.1.1.1 (127.1.1.1): 56 data bytes
ping: sendto: Can't assign requested address
^C

[mgraziano@monitor ~]$ sudo ifconfig lo0 alias 127.1.1.1 netmask 0xFFFFFFFF

[mgraziano@monitor ~]$ ifconfig lo0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
    options=3<RXCSUM,TXCSUM>
    inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3 
    inet6 ::1 prefixlen 128 
    inet 127.0.0.1 netmask 0xff000000 
    inet 127.1.1.1 netmask 0xffffffff 
    nd6 options=3<PERFORMNUD,ACCEPT_RTADV>

[mgraziano@monitor ~]$ ping 127.1.1.1
PING 127.1.1.1 (127.1.1.1): 56 data bytes
64 bytes from 127.1.1.1: icmp_seq=0 ttl=64 time=0.020 ms
^C

On the logic behind this implementation, see RFC 3330:

127.0.0.0/8 - This block is assigned for use as the Internet host
loopback address. A datagram sent by a higher level protocol to an
address anywhere within this block should loop back inside the host.
This is ordinarily implemented using only 127.0.0.1/32 for loopback,
but no addresses within this block should ever appear on any network
anywhere [RFC1700, page 5].

(emphasis mine)
Linux and Windows are being "helpful" here, however from my chair answering a request that was sent to an address not assigned to this host is not correct behavior...

I see the same behavior you describe on FreeBSD 8.1. Mac OS X, which shares some DNA with FreeBSD, also only seems to map 127.0.0.1.

Windows 7 and Linux (debian with 2.6.26 kernel) both appear to map the full address range as you describe in the Wikipedia quote (and as prescribed in the RFC).

To quote from RFC 3330:

127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network anywhere [RFC1700, page 5].

Depending how strictly you interpret the word "should," some might make the case that the FreeBSD/MacOS behavior is wrong. But given the ubiquitous use of 127.0.0.1 as the loopback address, I doubt it's likely to matter.