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When you see "unreachable" it measn that you can't find a route for that destination. (meaning : you don't have Gateway OR your Gateway can't find a route for that destination).
Time out: the destination is not responding. may be OFF or cable un plugged
this is happen when you are work on simulation software , when you are not configuring the protocol .
Because the TTL is predefine so the perticular protocol not responding or destionation host not reachable means the time for30 may complete still no connectivity happen . or router get off , temperory connectivity may also cause this problem .
Network Unreachable:- When you are try to ping unknown netwrok which is not avaliable in routing table then this message will appear
Host Unreachable:- This mesage will apear mostly if the host is down otherwise off-line. If the firewall is enable this message will appear.
Request time out:- this error message indicates that your host did not receive the ping message back from the destination device within the designated time period
network unreachable shows when there is something wrong in configuration in network of a pc or Ip addres assigning or subnet mask is not correctly assign.
Host unreachable means there Host is not connected to network or router or ethernet interface port wrongly assigned.
request timed out means it takes time for routing or there is no routing path for communicate with other Pc or there is no common path for route to communicate with others PC.But it requires that Same series of Ip can communicate without routing path.
If you see ICMP header you will find TYPE section which is1 Byte=8 bit.
8901 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | unused | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internet Header +64 bits of Original Data Datagram | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
So it means there may have2^8=255 categories of TYPE value
0 – Echo Reply
1 and2 Reserved
0 Destination network unreachable 1 Destination host unreachable 2 Destination protocol unreachable 3 Destination port unreachable 4 Fragmentation required, and DF flag set 5 Source route failed 6 Destination network unknown 7 Destination host unknown 8 Source host isolated 9 Network administratively prohibited 10 Host administratively prohibited 11 Network unreachable for TOS 12 Host unreachable for TOS 13 Communication administratively prohibited 14 Host Precedence Violation 15 Precedence cutoff in effect
4 – Source Quench
5 – Redirect Message
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30 – Traceroute
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40 Photuris, Security failures
41 ICMP for experimental mobility protocols such as Seamoby
42 through255 Reserved
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Now you can understand TYPE3 describes different categories destination unreachable:
Destination Host Unreachable: It is an error that specific host can not be reached.It means datagram is sent to the specific network but host is not found.It is a routing issue or default gateway is in different network.
Destination Network Unreachable: It is an error that specific network can not be reached.usually means a problem with routing but also can be caused by bad address.
Now you can understand TYPE0 describes about Echo:
Request Timed Out: You will see Request Timed Out when TYPE0/Echo is used and if there's no response.It means your gateway is okay but no response from destination host
For more info please click on the following link:
http://www.tcpipguide.com/free/t_ICMPv4DestinationUnreachableMessages-3.htm
"Network unreachable" always means a routing problem. Perhaps your gateway address isn't on the same subnet as your own IP address
DESTINATION HOST UNREACHABLE means that your computer doesn't know howto send packets to the destination address, because none of theentries in the route table match the destination. It usually meansthat there's a problem with the default route, which uses the defaultgateway address to communicate with all IP addresses that aren't onthe local area network.Request Timed Out means that the computer found a route to thedestination and sent the packets, but that there was no reply.
