Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

As the title states. I was trying to ping a bunch of servers whose existence I am not sure of. There are 10 servers in all. Two of them got ping timed out while the other eight have could not find host. The 2 timed out ones show an IP which times out too on pinging. I did a quick nslookup on these servers and they did not have any DNS entries. What is the difference between "could not find host" and "timed out" when pinging fails?

share|improve this question
add comment

3 Answers

up vote 7 down vote accepted

Pinging a computer is a lot like sending a piece of mail. Let's say I want to send a ping to google.com. The first thing I have to do is figure out where google.com is. For mail, you'd use a phonebook - This will translate a name into an address that the mail system can understand. Computers use what are known as DNS (Domain Name System) servers - the same thing that nslookup displays when you query a specific name. If ping can't find a given name in the DNS, then it returns Could not find host.

Alright, so we have an IP address, what now? Now we have to route it. An IP address is a lot like the street name - It gives you a specific place, but doesn't tell you where that street is (Well, you could keep that information in a massive table, but it would be HUGE, and also mostly useless. Why keep track of the coordinates of every road in every country when you only send mail to your grandmother?). There is additional information to fix this issue - a Zip code. You may not know where the street is, but you know that the mail system in that zip code will, and because zip codes are somewhat ordered (bear with me), you know how to get it to that zip code. With computers, this is called a MAC address. A computer always knows what MAC addresses it's connected directly to, and what IPs they know how to route. When a computer or router needs to send a packet, it looks up which MAC address is responsible for handling that IP, and then sends the packet along to that system. If there is no system responsible for that IP, it returns No route to destination. (Imagine trying to send mail to the MOON. The mailman would just look at you funny and hand it back.)

Now, if the mail gets all the way to the target, they still have to send a reply. Normally, this is quick and easy, and you can just measure the time between when you send the message and received the reply. But sometimes, the reply gets lost in the mail. Or maybe grandma is asleep, and forgot to check her mail. Either way, you don't receive a reply, even after waiting a whole week! Eventually you just give up. With computers, this is Request timed out.


As to your question for the computers without DNS entries, most routers use themselves as a DNS cache, and add fake DNS entries for the computers that request DHCP leases from them if the computer submits a hostname when requesting a DHCP lease. This is why it's sometimes possible to ping a computer's hostname. Windows systems also track NetBIOS names (usually, the hostnames of windows computers that are connected on the same subnetwork and have sharing enabled), and will try to resolve these to IP addresses if requested. I can't remember if this works with ping, but it does not work for nslookup, since nslookup is specificall for checking DNS entries, and not NetBIOS entries.

share|improve this answer
    
But, none of my servers had DNS entries. As in, all 10 did not have them and only 8 of them said "could not find host: while the other two successfully converted the hostname into an IP but timed out. –  Gutsygibbon Oct 18 '12 at 15:39
    
This article claims it is when a router cannot find a route in the routing table. blogs.technet.com/b/flaphead/archive/2005/09/13/410710.aspx –  EBGreen Oct 18 '12 at 15:41
2  
@EBGreen - I think that's right. My understanding has always been "could not find host" means it can't figure out where the packets go - "request timed out" means it did figure out where they go (or at least it thinks it did) but got no response. –  Shinrai Oct 18 '12 at 15:54
    
Shinrai, I'm going to stick with what you think since I was thinking the same. But, the fact that none of the 10 servers have DNS entires confused me. How can the timed out servers even figure out where to send the packets if there is no dns entry? –  Gutsygibbon Oct 18 '12 at 17:26
1  
aha, that extra bit about NETBIOS was precisely what I wanted to know. I failed at asking a question, oh god. lol. –  Gutsygibbon Oct 18 '12 at 17:34
add comment

Well, this has a little to do with the OSI model and a little to do with network schema.

Ping Timed Out usually means that something is blocking the ping from going through.

Device Not Found literally means Ping was able to go out and look for the device and it was unable to locate it.

Ping operates on layer one. If the network card is turned on and pinged it will be found. That's about all you can do, it verifies that the device is physically there. If the device is not found, there is no NIC where it is looking.

If it times out it is possible that something like a firewall is blocking it. Ping was unable to complete so it's possible something is there, but Ping cannot tell.

share|improve this answer
add comment

Timed out means the other machine's IP address is known, usually it was found through DNS, but you didn't get any responses to the ping requests you sent. That's usually because that other machine is configured not to respond to pings or because it has a firewall that's blocking the ICMP ping messages.

Could not find host means the other machine's IP address couldn't be found through DNS so there was no way to even try sending pings.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.