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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 ...
naïveRSA's user avatar
  • 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 ...
jonathanjo's user avatar
  • 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) ...
Arjan's user avatar
  • 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 ...
Martin Rosenau's user avatar
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 ...
JFL's user avatar
  • 19.7k
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. ...
boomi's user avatar
  • 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 ...
Ron Trunk's user avatar
  • 67.6k
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 ...
Ron Maupin's user avatar
  • 99.8k
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 ...
Peter Green's user avatar
  • 13.3k
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 ...
SuB's user avatar
  • 308
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 ...
Phil Frost's user avatar
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 ...
Ron Maupin's user avatar
  • 99.8k
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 ...
Ron Maupin's user avatar
  • 99.8k
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 ...
Ron Maupin's user avatar
  • 99.8k
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 ...
Jesse P.'s user avatar
  • 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 ...
Ron Maupin's user avatar
  • 99.8k
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. ...
jonathanjo's user avatar
  • 16.3k
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, ...
Winnie Tigger's user avatar
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/...
Benjamin Dale's user avatar
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 ...
Ron Maupin's user avatar
  • 99.8k
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 ...
Ron Maupin's user avatar
  • 99.8k
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 ...
jonathanjo's user avatar
  • 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 ...
JCRM's user avatar
  • 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 ...
Darrell Root's user avatar
  • 2,203
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 ...
Ron Maupin's user avatar
  • 99.8k
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 ...
Dave Noonan's user avatar
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 ...
vk5tu's user avatar
  • 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 ...
JFL's user avatar
  • 19.7k
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 ...
Zac67's user avatar
  • 84.9k

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