Mission
Ping the web server at 203.0.113.20 and watch the hop counter fall on the way there.
topology · the packet carries its own ttl live
ping · outputnot sent
$ ping 203.0.113.20
icmp_header · live values
- icmp type
- 8 — echo request
- code
- 0
- ttl on the wire
- 64
- identifier / seq
- 0x4f21 / 1
- carried inside
- ip.proto = 1
A question: are you reachable?
Code 0 is the only code these types use.
One less at every router; layer 2 devices leave it alone.
How the tool pairs a reply with the request that earned it.
ICMP sits directly on IP — no ports, no TCP, no UDP.
icmp_tool
Tool
IDLE
Echo request built (type 8)
Router decrements ttl 64→63
ISP decrements ttl 63→62
Server flips it to type 0
Reply matched — rtt measured
Think first · step 1 of 5
Ping wants to know whether 203.0.113.20 is reachable. What kind of message does it build first?
what just happened
Ping asks one question and times the answer. It is the smallest useful conversation two machines can have — and it uses no ports at all, because ICMP is not a transport protocol; it rides straight on IP.
ttl=64 leaving · arrives 61 · two routers on the path
Mission
Ping the web server at 203.0.113.20 and watch the hop counter fall on the way there.
Questions you will answer
- What happens to the TTL at every router?
- Which ICMP messages does ping use?
Read it now and keep it in mind — the answer is in what you are about to watch.