Private IP Addresses in VoIP Networks Considered Harmful: Sloppiness

Over the past few years, it's become very common for VoIP carriers
(i.e., telephone companies using VoIP) to use private (RFC 1918) IP
addresses in their internal VoIP network. These have IPs like or This practice has been promoted by many of
the hardware vendors. It reflects the setup many of them use in their

There are a number of problems this creates. One of the biggest
problems is really a human problem: people aren't nearly as careful
with private IP address space as they are with public Internet IP
addresses. So in several cases, including some very large firms,
they've accidentally re-used the same IP addresses on two different

The result is that the two different networks can't reach each other;
nor can you route through one of them to the other.

Every technical problem is, in a way, a human problem. The human part
of this is just outright sloppiness and laziness. In this case,
there's nothing fundamentally wrong with the private IPs — it's the
way the people are using them. Private IPs are traditionally used in
home networks and small-office networks. They're used very informally.
In many cases, a home or office network of private IP addresses can be
re-numbered easily by changing the DHCP server.

In VoIP networks, though — as in all server networks — renumbering
is a big deal. DNS should aid in renumbering. But many VoIP devices
don't really use DNS to find each other — they use IP addresses, and
don't support DNS lookups. (The reasons for this appear to be a lack
of faith in reliable DNS operation, and a tradition of using numeric
point codes in the SS7 network.)

VoIP is hard enough as it is. Carrier VoIP is harder than enterprise
VoIP. Bringing along the whole mentality of slap-'em-on-there private
IPs just doesn't help anybody.


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