47
votes
Why can I traceroute to this IP address, but not ping?
On a similar question here Luke Savage explained it perfectly:
Traceroute is not a protocol itself, it is an application and the protocols used depends on the implementation your are using. Primarily ...
- 760
23
votes
Accepted
Where is Ping's "round-trip time" stored in the IP header?
The round trip time is not actually stored anywhere. The sending host remembers the time it sends each ICMP Echo Request message, using ICMP's 16-bit identifier and sequence fields. When it gets the ...
- 16.2k
19
votes
Why can I traceroute to this IP address, but not ping?
To add to @naïveRSA's answer, if there's filtering/firewalling in the path one could also have the situation where an ICMP "echo reply" (ping) packet is blocked, but an ICMP "time exceeded" (tracert) ...
- 331
18
votes
Ping port number?
I'd like to give you an additional answer especially to this part of the question:
... someone says ICMP uses Port 7
Port 7 (both TCP and UDP) is used for the "echo" service.
If this service is ...
- 2,346
14
votes
Accepted
What would happen to Ping if two PCs have same IP address with same Subnet Mask on same network?
Actually, the ping gets sent to a layer-2 address if they are on the same LAN. Assuming ethernet, the sending host may have a MAC address in its ARP cache, and the pings gets sent to the host with ...
- 98.7k
11
votes
Accepted
Ping a private IP address in another network
A PC in a private IP range can't be acccesed from the public internet.
Devices in private range connecting to internet use a proxy or router/NAT device that replaces the local source IP for a single ...
- 4,487
11
votes
Accepted
What happens with a ping when it enters a network? Is the router the one answering with a reply?
What happens with a ping when it enters a network? Does the router redirects it to one of the devices on the network, or it sends the reply itself?
A router routes packets towards the destination. An ...
- 82.6k
9
votes
Accepted
Difference between ping timed out and destination host unreachable
First we need to understand how packets are sent.
When a host or router tries to send an IPv4 packet* it first looks up the destination in it's routing table. Based on the routing table it determines ...
- 13.1k
9
votes
Why can I traceroute to this IP address, but not ping?
Let's look at what happens, shall we?
8.8.8.8 makes a good example, because at least from my location, I can reach it both with traceroute and ping.
First let's try ping 8.8.8.8 and watch what ...
- 321
8
votes
Accepted
Strange router's MAC address from ICMP Echo Reply?
You are confused. What you claim is your router's MAC address is not your router's MAC address. The MAC address is in the range, 0000.0C07.ACxx, which is the MAC address range for HSRP. The 06 on the ...
- 98.7k
8
votes
Difference between ping timed out and destination host unreachable
The Request timed out. is while the destination MAC address is still in the source/router's ARP cache. The Reply from 192.168.2.120: Destination host unreachable. is after the destination MAC address ...
- 98.7k
8
votes
Why ping with source address?
The biggest reason is that the return path from the destination back to you may depend on the source address. Often you want to test the network path in both directions.
Furthermore things like ...
- 13.1k
8
votes
Ping port number?
As others have already stated, in general pings are ICMP-based and have no ports. There is, however, such a thing as TCP Ping where, instead of the typical 3-way TCP handshake, only the first 2 steps ...
- 4,690
8
votes
Ethernet padding
IEEE 802.3 describes structure of Ethernet frames. As it says the minimum frame length is 64 bytes. Every frame less than 64 bytes should be padded with 0 before transmitted on the Ethernet link. This ...
- 298
8
votes
Accepted
Why can I ping IP addresses from 127.0.0.1 to 127.255.255.254?
The entire address block 127.0.0.0/8 is the block of loopback addresses for a host. There are RFCs that explain this.
The goes back at least as far as RFC 990, ASSIGNED NUMBERS:
The class A network ...
- 98.7k
8
votes
Accepted
does the DSL distance really add more latency
Twisted pair cable used for the local loop has a velocity factor of about .58 - each km of cable takes ca. 6 µs to travel, adding ca. 12 µs to latency. The rest of the Internet probably uses fiber ...
- 82.6k
8
votes
When is 'Timestamp' and 'Timestamp Reply' are used in ICMP protocol?
The usual ping command uses ECHO REQUEST and ECHO REPLY, as you've seen. It does indeed locally keep track of sent time and matches with the incoming reply to determine the round trip time.
...
- 16.2k
7
votes
Accepted
Ping in the same subnet
On a directly connected layer-2 network/segment, IP communication works by encapsulating IP packets into layer-2 frames addressed to the destination's MAC. The source learns the destination's MAC ...
- 82.6k
7
votes
Accepted
Why I have higher ping to the VLAN interface than to other local interfaces
A "L3 switch" will perform the actual packet forwarding (both L2 and L3) using dedicated hardware, but exceptional cases like sending a ping reply or a time exceeded message are normally handled in ...
- 13.1k
7
votes
Accepted
Confusion about MTU
From reading on the web I understand that 28 bytes is reserved for
ICMP so my actual MTU is 1472.
No, the MTU is size of the payload of the data-link protocol. For ethernet, that is 1500. You are ...
- 98.7k
6
votes
Multiple NICs, only eth0 can ping
You have all your interfaces in the same subnet. That is not going to work. Your computer can't figure out which interface to use to send to each host because it believes they are reachable from any ...
- 67.1k
6
votes
Why can't two hosts (in two different logical networks) connected to each other through cross-over cable communicate?
A host knows if the destination address is on a different network (the same way that you do, by masking the host and destination addresses with the host mask). If the destination is on the same ...
- 98.7k
6
votes
Accepted
Understanding hubs and switches
If I connect 3 computers to the same hub, and use PC1 to ping PC2, then all the 3 computers will receive the ping?
PC2 and PC3 receive the echo request, and PC1 and PC3 receive the echo reply. A ...
- 82.6k
6
votes
What happens with a ping when it enters a network? Is the router the one answering with a reply?
It's the host which is addressed in the outgoing ECHO REQUEST packet which is responsible for answering. As far as the routers in between are concerned, this is just like any other IP packet, and ...
- 16.2k
6
votes
Accepted
Cisco, Router can communicate; hosts cannot
For NAT to work, you must specify a standard access list to identify the traffic that should be NATted. You reference the access list (source list 1) correctly, but you have not created the access ...
- 1,949
5
votes
Accepted
How are ping times computed
I'm not sure why you would think fragmentation would double the time. If a router fragments a packet, it sends both fragments sequentially. The host on the other end will get the fragments, reassemble ...
- 98.7k
5
votes
Multiple NICs, only eth0 can ping
If You need all nodes to work in one subnet and one L2 broadcast domain,
add bridge interface, enable rstp on it, assign IP on bridge interface and add physical interfaces to bridge.
/etc/sysconfig/...
- 774
Only top scored, non community-wiki answers of a minimum length are eligible