From owner-freebsd-net@FreeBSD.ORG  Sat Mar 17 17:09:32 2007
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Date: Sun, 18 Mar 2007 04:09:22 +1100 (EST)
From: Ian Smith <smithi@nimnet.asn.au>
To: Colin Waring <freebsd@southportcomputers.co.uk>
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Cc: freebsd-net@freebsd.org
Subject: Re: Troubleshooting aliases.
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On Sat, 17 Mar 2007, Colin Waring wrote:

 > Basically, .a and .d respond to pings and pass all traffic
 > .b and .c respond to pings but don't appear to pass any other traffic.
 > 
 > IPF is compiled but I've completely turned it off for testing

If you run one tcpdump on lo0 and another on em0 and try 'other traffic'
you should be able to see which packets are/n't getting out, or through. 

Other things maybe worth checking:

# netstat -finet -rna	# after pinging all your aliases
# arp -an		# check address (incl bcast) at MAC on interface
# sockstat -4		# which addresses/ports are listening/connected

 > None of the actual configuration has changed though so I wouldn't expect
 > anything to show up in ifconfig any as it was all working like this
 > previously..
 > 
 > em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
 >         options=b<RXCSUM,TXCSUM,VLAN_MTU>
 >         inet6 fe80::215:c5ff:fe5d:f7b7%em0 prefixlen 64 scopeid 0x1
 >         inet a.a.a.a netmask 0xffffff00 broadcast a.a.a.255
 >         inet a.a.a.b netmask 0xffffffff broadcast a.a.a.255
 >         inet a.a.a.c netmask 0xffffffff broadcast a.a.a.255
 >         inet a.a.a.d netmask 0xffffffff broadcast a.a.a.255
 >         ether 00:15:c5:5d:f7:b7
 >         media: Ethernet autoselect (100baseTX <half-duplex>)
 >         status: active

Did you specify those broadcast addresses on b, c and d?  Here I'd see
a.a.a.b through a.a.a.d broadcast addresses for those having 0xffffffff
netmasks (on 5.5-S) but don't know if that's relevant to your problem. 

Cheers, Ian