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

Terje Elde terje at
Tue Oct 13 08:10:13 UTC 2015

> On 13 Oct 2015, at 09:06, Matthew Seaman <matthew at> wrote:
> 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.

Using my ISP as an example, they’re operating their DHCP-server on  It’s actually quite common.

As for “just layer-2 addressing”, I’m not sure about your wording, as there’s always L3 involved, just broadcast ones.  I do get your point though, and it’s valid for initial setup, but using direct unicast between server and client is fine once they’ve established an IP.  For things like renewals.

> 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.

I’ve never seen or heard of an ISP that informs their customers of this.  In my case, it’s one of Norways largest cable providers.

I’m not saying it’s probably in this case, just nice to rule out, or take the cheap “Don’t worry about it.” if it is the case.


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