You can ping the broadcast address, try it

I am not aware of any routing protocol that allows one to route actual useful data packets to a broadcast address. Put another way how does the router route a packet to the network broadcast address? The next address range up (91.0.0.0/12) is owned by Deutsche Telekom AG so Vodafone are not combining net assignments.

You can test this quite simply I can ping the broadcast address, but the moment I try and connect for a DNS query I get a no route to host error.

[root@buzzard ~]# ping -c 3 90.255.255.255
PING 90.255.255.255 (90.255.255.255) 56(84) bytes of data.
64 bytes from 90.255.255.255: icmp_seq=1 ttl=54 time=6.65 ms
64 bytes from 90.255.255.255: icmp_seq=2 ttl=54 time=6.56 ms
64 bytes from 90.255.255.255: icmp_seq=3 ttl=54 time=6.62 ms

--- 90.255.255.255 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 6.562/6.613/6.652/0.037 ms
[root@buzzard ~]# telnet 90.255.255.255 53
Trying 90.255.255.255...
telnet: connect to address 90.255.255.255: No route to host

It looks like a mistake from Vodafone to me. I guess they could have done something special with their routers to make it work but it would not be standards compliant I think. Technically I think a UDP request on port 53 to 90.255.255.255 might be forwarded to all hosts the network with one or more responding. However it's highly none standard and would like break a lot of software not least because you can perfectly legitimately do DNS over TCP.

Personally all I take from my ISP is my IP address (which is actually fixed), network and gateway. The rest they can take a running jump.

Easiest just change it to something else anyway IMHO.

I've just used an internet tracert tool and can trace the route to 90.255.255.255 just fine. Are you sure it isn't the difference between LAN and WAN - you can do broadcast packets over a LAN but I am not sure you can over a WAN - the fact these are public WAN IPs means that the routers likely don't treat it as a broadcast address as it wouldn't make any sense to do so - therefore the address is available for allocation.

In reply to a post by jabuzzard:

You can ping the broadcast address, try it

That works better if you have a unix or linux based OS (e.g. macOS) than on Windows

90.255.255.255 is not a broadcast address (whois, using a tool which knows how to lookup IP assignments not just domains).

Vodafone have assigned all of 90.255.255.0/24 to anycast services, so a connection to an IP in this range is routed to the "best" of a group of hosts based on topology / geography or any other reasonable criteria (the exact method does not matter).

I won't go into a full explanation of anycast addressing here (one can readily find technical descriptions) suffice to say, for a client it won't really see any difference from connecting to a unicast address.

BTW just because an IP is pingable doesn't dictate which other protocols or services a given host will be offering or be reachable though the provider's firewall.
And telnet, while possible to hand-craft connecting to services which reply using human-readable text such as SMTP, will really only tell you about connecting to TCP ports which is not what vanilla DNS* uses . There are more generic tools for TCP/UDP and other protocols (e.g. nc, nmap and "port scanners").

Although I am arbitrarily replying to a post, this is more a general reply to the thread as a whole.

(*) not withstanding extensions such as EDNS and DNSSEC which do!

Edited by prlzx (Tue 22-Oct-19 22:00:28)

I'm currently with Vodafone FTTC. Vodafone's DNS servers go down regularly.
So I manually set my router to use google DNS 8.8.8.8 and 8.8.4.4
Never had a DNS problem since then.