DNS – Can Two Hostnames Share the Same IP Address?

domain-name-systemhostnameip

Can anyone explain whether it is possible for two hostnames to share the same IP address?

And what about if one hostname represents more than one IP address, is that possible too? Why?

Best Answer

Assigning more than one IP address to one hostname is also possible:

rr.example.com.        A      192.0.2.12
rr.example.com.        A      192.0.2.23
rr.example.com.        A      192.0.2.34
rr.example.com.        A      192.0.2.45

When you query a DNS server for rr.example.com you'll get back a list of IP addresses back. You can then choose to connect to one of them. Should the first attempt to connect get actively refused, just try the next.

Most browser will follow this flow, as long as the endpoints actively refuse TCP connectivity. Should an endpoint timeout, the ressource will be treated as unreachable even though not all IP's has been tried

Since most applications (browsers included) are often only interested in 1 IP endpoint at a time and just choose the first available answer, you risk skewing the load between the target servers so that the first server gets all the traffic while the others might be idle.

To circumvent this, most DNS servers offer what is known as a Round Robin configuration, making the server alternate the order in which equally matching records are returned. Before load balancers were commonplace, this was an efficient way to load balance and somewhat implement fault-tolerance on network systems.

Related Solutions

Linux – Is it possible to alias a hostname in Linux

For those who don't have an account on the forums (or don't wish to login):

if your main issue is not to ping but to ssh, you can create/edit your ~/.ssh/config adding lines like these:

Host fakehost1
  Hostname real-hostname

Host fakehost2
  Hostname real-hostname2

Host fakehost3
  Hostname real-hostname3

Difference Between IP Address 0.0.0.0 and 127.0.0.1

The only thing is that you're not saying "all addresses should have access" -- that's done in your firewall(s) and/or the server software and/or other security layers like tcpwrappers.

0.0.0.0, in this context, means "all IP addresses on the local machine" (in fact probably, "all IPv4 addresses on the local machine"). So, if your webserver machine has two IP addresses, 192.168.1.1 and 10.1.2.1, and you allow a webserver daemon like apache to listen on 0.0.0.0, it will be reachable at both of those IP addresses. But only to what can contact those IP addresses and the web port(s).

Note that, in a different context (routing) 0.0.0.0 usually means the default route (the route to "the rest of" the internet, aside from routes in your local network etc.).

Best Answer

Related Solutions

Linux – Is it possible to alias a hostname in Linux

Difference Between IP Address 0.0.0.0 and 127.0.0.1

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