No route to host : ssh Error

Question

I have a laptop and a PC ([email protected]) , I usually connect the laptop to PC via ssh. But now when I try ssh -X [email protected] , it is showing the following error ssh: connect to host 172.16.13.151 port 22: No route to host. What could be the possible problem ?

Output of ifconfig is the following:

eth0
Link encap:Ethernet  HWaddr 2c:41:38:0d:bb:a2
UP BROADCAST MULTICAST  MTU:1500  Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000 
RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

lo
Link encap:Local Loopback
inet addr:127.0.0.1  Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING  MTU:65536  Metric:1
RX packets:1750 errors:0 dropped:0 overruns:0 frame:0
TX packets:1750 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0 
RX bytes:134949 (134.9 KB)  TX bytes:134949 (134.9 KB)

wlan0
Link encap:Ethernet  HWaddr d0:df:9a:76:18:1e
inet addr:172.16.101.213  Bcast:172.16.111.255  Mask:255.255.240.0
inet6 addr: fe80::d2df:9aff:fe76:181e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:595855 errors:0 dropped:0 overruns:0 frame:0
TX packets:37917 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000 
RX bytes:138691381 (138.6 MB)  TX bytes:4733835 (4.7 MB)

output of route is the following :

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         172.16.96.1     0.0.0.0         UG    0      0        0 wlan0
link-local      *               255.255.0.0     U     1000   0        0 wlan0
172.16.96.0     *               255.255.240.0   U     2      0        0 wlan0

Output of traceroute 172.16.13.151 gives

traceroute to 172.16.13.151 (172.16.13.151), 30 hops max, 60 byte packets  
1  172.16.96.5 (172.16.96.5)  456.381 ms  457.461 ms  457.448 ms       
2  172.16.96.5 (172.16.96.5)  5007.656 ms !H  5007.665 ms !H  5015.906 ms !H

Solution

The source 172.16.101.213 and the destination 172.16.13.151 are not on the same network (based on the netmask 255.255.240.0). Thus your laptop is sending the packets to the default gateway 172.16.96.1, which apparently does not know the route to your PC.

Comments

[Admin]

You can diagnose this further with traceroute command. Most likely your gateway router 172.16.96.1 is missing the route to 172.16.13.0/24 network.

[Admin]

Please edit debug output like this always into the question, it is difficult to read from the comments.

[Admin]

I've modified the question including the traceroute part

[Admin]

@TeroKilkanen If it was missing the route it shouldn't take 5 seconds to respond. More likely it does have a route and sends ARP queries for the destination. But nobody answers the ARP queries so after 5 seconds it times out.

[Joe]

@Johan MyréenI think you are making a mistake here https://en.wikipedia.org/wiki/IPv4_subnetting_reference, both source and destination are on the same network here . Subnet mask is 255.255.240.0 not 255.255.255.0, both are different. Here the default gateway is the IP of the router and it should be able to find out the PC. Please correct me if I am wrong. I also don't think taht host and the destination has to be on the same network inorder for SSH connections.

[Johan Myréen]

In hex notation the netmask is ff.ff.f0.00, the source address is ac.10.65.d5 and the destination address ac.10.0d.97. The first five hex digits should be equal for the addresses to be on the same subnet.

[Joe]

@Johan Myréen Thank you for correcting my mistake. But it is not necessary that the host and the destination should be on the same network inorder for ssh connection, right ?

[Johan Myréen]

No, of course it's not necessary for the the source and destination to be on the same subnet, otherwise ssh would not go very far. I jumped to this conclusion based on that you had two similar addresses and just a default route in addition to that. Btw, what is the 172.16.96.5 address in your traceroute listing above?

[Joe]

@Johan Myréen Actually I'm using my college wifi here and the routers are DHCP servers. So the gateway changes as I move around.

[Johan Myréen]

Ok, here's one more wild guess: maybe the subnetworks don't "see" each other, and your PC and laptop were originally on the same subnetwork by pure luck.