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 ...
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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) ...
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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 ...
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14
votes
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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 ...
- 98.8k
12
votes
Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page?
The original RFC for ICMP, RFC777 state that:
ICMP, uses the basic support of IP as if it were a higher level
protocol, however, ICMP is actually an integral part of IP, and must
be implemented by ...
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11
votes
Accepted
Why there are ip(ip) and domain(ip) 's difference and multi rows records in the traceroute?
1) Traceroute may attempt to resolve the hop's domain name via DNS. As you didn't include any additional flags to explicitly enable this functionality, your traceroute application does so by default. ...
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10
votes
Why doesn’t ICMP use port numbers?
Port numbers are a feature of transport layer protocols such as TCP and UDP. ICMP is really part of the internetworking layer (IP), so that attribute doesn't exist at that layer.
And yes, I know ...
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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 ...
- 13.2k
9
votes
Does a router send frames or packets?
it seems that routers decapsulate the frame on arrival, and
encapsulate the packet in a frame in order to send it.
Yes. A router must strip off the layer-2 frame in order to get to the layer-3 ...
- 98.8k
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 ...
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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 ...
- 98.8k
8
votes
Accepted
Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page?
There is much controversy about which protocols run at which layers in the models. Understand that the models, e.g. OSI Model, are just models, and the real world is often different.
There are ...
- 98.8k
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 ...
- 98.8k
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 ...
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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
8
votes
Why doesn’t ICMP use port numbers?
ICMP is IP protocol 1, TCP is 6 and UDP is 17. So these are all different protocols with different behaviors.
TCP and UDP have Ports, ICMP does not.
TCP and UDP have an Echo Service on Port 7, ...
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8
votes
Accepted
Are ICMP Redirect messages really bad?
ICMP re-directs are most often seen when you have a host or router A in the same subnet with two other routers B & C and connectivity to both. Consider the following network:
|__192.168.1.0/...
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7
votes
ICMP packet with correct mac address but wrong ip address
It is as you said. The computer with that MAC address will receive the ethernet frame, since on layer 2 it's all good. Then, looking at layer 3, it will discard the IP packet, since the IP address is ...
- 17.1k
7
votes
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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 ...
- 98.8k
6
votes
Accepted
Features of ICMP
ICMP message is not generated on header checksum errors
The source address to which ICMP would send a message is in the IP header, and if the header has an error, the source address in the header ...
- 98.8k
6
votes
Accepted
Whether the traceroute command only can check "to" path, can not check "back" path?
Traceroute, by its nature, only reports on the path towards the target, and very imperfectly too.
You need to run traceroute from both sides to see if your routes are asymmetric.
It has to be ...
- 16.2k
6
votes
Are ICMP Redirect messages really bad?
ICMP redirect is a remnant from an era of trust - partly because networked machines had administrators and BYOD was unimaginable.
Ignoring the redirect on the client means it will continue to be sent ...
- 161
6
votes
Accepted
Why ICMP Destination Port Unreachable error messeage is generated for unreliable UDP packets?
From ICMP RFC 792:
If, in the destination host, the IP module cannot deliver the
datagram because the indicated protocol module or process port is
not active, the destination host may send a ...
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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 ...
- 98.8k
5
votes
Accepted
Default Gateway Best Practice
A very quick (and therefore incomplete) answer:
Assuming all of the inside stuff is on the same subnet the ICMP messages are switch2 saying "I don't need to be involved in this, talk to the firewall ...
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