Are udp packets with non-routeable ip addresses valid on public network?

Matthew Seaman matthew at
Tue Oct 13 07:06:22 UTC 2015

On 12/10/2015 19:32, Terje Elde wrote:
> On 12. okt. 2015, at 15:06, Ernie Luzar <luzar722 at> wrote:
>>> I am receiving unsolicited inbound udp packets with a "to ip
>>> address" [] of a computer on my LAN. Is this valid?

> It shouldn't be routes over the internet to you, but it's pretty
> common to use between your ISP and yourself, for things like DHCP.
> What are the port-numbers? 67/68 or thereabout?

Uh... DHCP usually uses just layer-2 (ie. Ethernet) addressing which is
a useful feature in a protocol designed to specify host IP addresses...
In those cases where you can't have a DHCP server on the same ethernet
segment, there is a specific default IP address range for a client host
to fall back to in order to make a layer-3 connection to a DHCP server
-- which is the link-local address range.  See RFC 5735.

If his ISP was using addresses for their customer-facing
network segments, I'm pretty sure the OP would have been told about it
and hence not be alarmed at seeing that traffic.



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