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 ...
- 16.3k
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 ...
- 2,356
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. ...
- 993
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 ...
- 67.9k
10
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 ...
- 100k
9
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 ...
- 308
9
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.
...
- 16.3k
9
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/...
- 9,460
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 ...
- 331
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
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 ...
- 100k
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, ...
- 286
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 ...
- 100k
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 ...
- 100k
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 ...
- 16.3k
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 ...
- 2,203
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 ...
- 950
5
votes
Accepted
How do i clear icmp statistics (JUNOS)
show system statistics icmp shows the involvement of the Route Processor in handling ICMP. It's a useful capacity planning tool for setting filters to avoid the RP being attacked by ICMP floods. The ...
- 1,171
5
votes
Accepted
How do ICMP packets travel in a networks?
It is indeed specified in RFC 792, on the top of page 2:
Message Formats
ICMP messages are sent using the basic IP header.
You will also found in this RFC :
ICMP, uses the basic support of IP as ...
- 19.8k
5
votes
Accepted
CRC done by ICMP?
No.
ICMP is used to send IP layer notifications between hosts. The IPv4 header has got a header checksum, as do many transport layer protocol headers.
TTL is a method to keep packets from ...
- 86.4k
5
votes
Accepted
Which first eight bytes of the IP datagram are included in ICMP message?
Well, the sender might like to know which ones of its packets the ICMP message refers to. For that, the entire IP header plus (at least) the first eight bytes of the IP payload are appended to the ...
- 86.4k
5
votes
Accepted
How to use ICMP to send message
Ping doesn't send user messages. It generates an ICMP Echo Request that the destination is supposed to answer with an ICMP Echo Reply.
Echo requests use a dummy payload data encapsulated within the ...
- 86.4k
5
votes
Accepted
ICMP distinguish error messages
In many cases, an ICMP error message contains the header of the packet that caused the error.
Example: The ICMP Type3 Code 3 "Port Unreachable" response from a "server" (172.19.41....
- 8,846
5
votes
Accepted
What does a router do with a packet if it can't find the destination IP address in its routing table?
ARP is a layer-2 thing -- it's how layer-2 address (MAC) is associated with a layer-3 address (IPv4). The router (any node) will only respond to ARP requests for addresses it owns -- the addresses ...
- 32.4k
5
votes
Accepted
If ICMP packet is lost then another ICMP could be generated?
ICMP errors are NOT generated for ICMP errors. (that's the recipe for a packet storm)
If a router is dropping a frame due to no buffer space, no error will be generated because it never had the packet ...
- 32.4k
4
votes
Accepted
Ping Port Number
Port numbers are layer-4 addresses for some layer-4 protocols, e.g. TCP or UDP. Other layer-4 protocols may use something else, or nothing at all.
ICMP does not use port numbers. ICMP acts like a ...
- 100k
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