Why use private network addresses?
It happens less and less, but I still see systems where people
have used public ip addresses for private networks. These systems
sometimes even end up behind a NAT firewall and while they "work",
problems are out there.
There are several ranges of IP addresses that you SHOULD use for
internal, private networks:
- 10.0.0.1 through 10.254.254.254
- 172.16.0.1 through 172.31.254.254
- 192.168.0.1 through 192.168.254.254
This is the subject of RFC-1918, Address Allocation for Private Internets
Let's say you ignore this and just arbitrarily pick some other
scheme for your internal network. You have a firewall that happily
maps your internal addresses to a real public address, so things do
work - until you need to access a site that actually has the
address range you incorrectly used for your network. Your packets
won't be routed to that site at all, because they are seen by your
router as being local.
The opposite side of this is that if you do use the reserved private IP
addresses, you can't look them up on the internet to resolve host
names. You need to resolve those locally, not look to your ISP's
Why is THAT important? Well, programs like telnetd will try to
do a reverse DNS lookup when your local machines try to connect.
This is often a cause for slow telnets on the local network.
From Telnet/FTP is very slow to connect: Slow telnet or ftp connections are
often caused by the server wanting to do a reverse DNS lookup to
find out who is connecting. If you aren't running DNS, you can fix
this just by listing all the machines in /etc/hosts. Note that you
don't have to be accurate about the names: I often use the ip
adress with "_" substituted for the "."'s, like "host_192_168_2_3"
and so on. A simple script:
while [ $x -lt 255 ]
echo "192.168.2.$x host_$x"
x=$((x + 1 ))
done >> /etc/hosts
Understand that being slow to give up on name resolution is an annoyance on small networks and a Good Thing on large networks.
Systems that give up quickly work well on small networks, but don't
get the information they should have on larger nets.
See Networking 101.
Do you need to renumber?
Are you facing a renumbering problem? Are you looking
at renumbering and envisioning horrible pain and massive downtime?
I have a customer using a 210. scheme, so I've been a-googling. - I'm looking for a way this customer doesn't have to pay the piper in full just yet. I haven't found much. I found Issues with Private IP Addressing in the Internet which might offer some illumination or motivation, but no help to make it easier..
It's a nasty problem..
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© 2013-08-13 Tony Lawrence