====== Traceroute ======

**Traceroute** is a network diagnostic tool that discovers the path packets take from your computer to a destination on the Internet.

Unlike Ping, which only answers:

> "Can I reach the destination?"

Traceroute answers:

> "How do I reach the destination?"

It identifies each router (called a **hop**) along the path.

-----

===== Why Use Traceroute? =====

Traceroute helps diagnose problems such as:

  * Routing failures
  * High network latency
  * Packet loss
  * ISP routing issues
  * Network congestion

It is one of the most useful tools for troubleshooting Internet connectivity.

-----

===== How Traceroute Works =====

Traceroute sends packets with gradually increasing **Time To Live (TTL)** values.

Example:

<code>
TTL = 1

↓

Router 1

↓

ICMP Time Exceeded
</code>

Then:

<code>
TTL = 2

↓

Router 1

↓

Router 2

↓

ICMP Time Exceeded
</code>

Then:

<code>
TTL = 3

↓

Router 1

↓

Router 2

↓

Router 3

↓

ICMP Time Exceeded
</code>

This continues until the destination responds.

Each router reveals its address by sending an **ICMP Time Exceeded** message.

-----

===== Example Output =====

A typical traceroute might look like:

<code>
1   192.168.1.1
2   10.25.0.1
3   185.229.29.1
4   172.217.20.46
</code>

Each line represents one **hop**.

The final hop is usually the destination.

-----

===== What Is a Hop? =====

A **hop** is one router that forwards your packet.

Example:

<code>
Your Computer
        │
Hop 1
Home Router
        │
Hop 2
ISP Router
        │
Hop 3
Regional Router
        │
Hop 4
Destination
</code>

The number of hops depends on the network path.

-----

===== Round-Trip Time =====

Traceroute usually measures the response time for each hop.

Example:

<code>
Hop  RTT

1    1 ms

2    8 ms

3    21 ms

4    22 ms
</code>

A sudden increase may indicate:

  * Network congestion
  * Long-distance routing
  * Slow routers

-----

===== Why Are Three Times Usually Shown? =====

Many traceroute implementations send **three probes** for each hop.

Example:

<code>
5

24 ms
25 ms
23 ms
</code>

Multiple probes help identify:

  * Temporary congestion
  * Packet loss
  * Variable latency (jitter)

-----

===== What Does "*" Mean? =====

Sometimes a hop is displayed as:

<code>
7   * * *
</code>

This usually means the router did not reply.

Possible reasons include:

  * ICMP filtering
  * Firewall rules
  * Rate limiting
  * Timeout

A missing reply **does not necessarily mean the router is broken**.

If later hops respond normally, the router is still forwarding traffic.

-----

===== Traceroute vs Ping =====

^ Ping ^ Traceroute ^
| Tests reachability | Discovers the network path |
| Reports overall latency | Reports latency at each hop |
| Uses ICMP Echo | Uses increasing TTL values |
| Does not identify routers | Shows intermediate routers |

These tools complement each other.

-----

===== Different Implementations =====

Different operating systems use different probe types.

^ Operating System ^ Typical Probe Type ^
| Windows (tracert) | ICMP Echo Request |
| Linux (traceroute) | UDP (default) |
| Linux (optional) | ICMP or TCP |
| macOS | UDP (default) |

Although the probe types differ, the principle remains the same.

-----

===== Traceroute and Telenegar =====

**Telenegar Trace Route** performs traceroute from multiple probe locations.

This allows you to compare routing from different ISPs and regions.

For each probe, Telenegar may report:

  * Hop number
  * Router IP address
  * Hostname (when available)
  * Response time
  * Packet loss
  * Geographic location (when available)

This makes it easier to determine whether a routing problem is:

  * Local
  * ISP-specific
  * Regional
  * Global

-----

===== Example Investigation =====

Suppose users report that a website is slow.

Ping shows:

<code>
Destination reachable

RTT = 120 ms
</code>

Traceroute shows:

<code>
Hop 1   1 ms

Hop 2   6 ms

Hop 3   9 ms

Hop 4   110 ms

Hop 5   112 ms
</code>

This suggests that the delay begins near **Hop 4**, helping identify where the problem occurs.

-----

===== Common Misconceptions =====

  * **Traceroute always shows every router.**
    * False. Some routers do not respond to traceroute probes.

  * **A timeout means the network is broken.**
    * False. Many routers intentionally ignore traceroute requests.

  * **Traceroute always uses ICMP.**
    * False. Depending on the operating system, it may use ICMP, UDP, or TCP probes.

-----

===== Related Topics =====

  * [[networking:diagnostics:ping|Ping]]
  * [[networking:protocols:icmp|ICMP]]
  * [[networking:internet:default-gateway|Default Gateway]]
  * [[tools:traceroute|Trace Route]]

-----

{{tag>traceroute tracert icmp ttl diagnostics networking routing}}