إجابة مرجعية
Here's what you need to remember: A ping command is given because it is the simplest way to check if a system is reachable over a network
But how does it work?
So, when you run a ping, your machine sends an ICMP Echo Request to the destination. If the destination is reachable, it replies with an ICMP Echo Reply.
And so, ping is responsible for 2 things, and those are if the system is reachable or just how long it would take, i.e, the round-trip time.
Now, along with this, every packet also carries something called TTL, i.e, Time To Live.
TTL is just a counter inside the IP packet.
Each time the packet passes through a router, the TTL is reduced by 1. When it reaches 0, the packet is discarded, and the router sends back an ICMP ‘Time Exceeded' message.
You might be wondering what is the need of these
So, essentially, because without TTL, a packet stuck in a routing loop could keep circulating forever. And that is why TTL helps packets eventually expire.
One thing you probably would notice in ping output is the TTL value.
Different systems use different default TTLs, for example, Linux/macOS - around 64 and Windows - around 128.
So sometimes, you can roughly guess the OS based on the TTL in the reply.
When we look at traceroute and how it connects:
traceroute cleverly uses TTL.
It sends packets with TTL = 1, then 2, then 3… Each router drops the packet when TTL becomes 0 and replies back.
This is how traceroute maps the path from source to destination.
And voila, this is everything you can cover for a ping-related question, but yes, there can be a follow-up question in the interview, like:
Q. If ping works but HTTP doesn't, what does that mean?
Your ans: It means basic network connectivity is fine. The issue is likely at a higher layer, for example, a blocked port, a service not running, or an application-level problem.
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