TH4. Telnet vs Ping

Telnet vs Ping in OCI

Overview

In OCI Networking troubleshooting, both Ping and Telnet are important tools, but both have different purposes.

Feature	Ping	Telnet
Purpose	Checks network connectivity	Checks specific port/service connectivity
Protocol Used	ICMP	TCP
Checks	Whether the server is reachable	Whether a specific service port is open
Port Required	No	Yes
Common Usage	Basic network testing	Service/port testing
OCI Requirement	ICMP must be allowed	TCP port must be allowed

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1. Ping in OCI

What is Ping?

The ping command is used to check whether the destination server or IP address is reachable over the network.

It sends ICMP Echo Request packets and waits for a response.

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OCI Example of Ping

Suppose:

• Instance A IP: 10.0.0.10
• Instance B IP: 10.0.1.20

From Instance A:

ping 10.0.1.20

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Successful Ping Output

PING 10.0.1.20 (10.0.1.20) 56(84) bytes of data.
64 bytes from 10.0.1.20: icmp_seq=1 ttl=64 time=1.23 ms
64 bytes from 10.0.1.20: icmp_seq=2 ttl=64 time=1.10 ms

Meaning

• Network connectivity exists
• Routing is correct
• ICMP traffic is allowed

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Ping Failure Example

Destination Host Unreachable

OR

Request timed out

Possible Reasons

• Security List blocks ICMP
• Route Table issue
• Instance is down
• Wrong subnet configuration
• Firewall blocks ICMP

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OCI Requirement for Ping

ICMP must be allowed in:

• Security List
• NSG (Network Security Group)
• OS Firewall

Example Security Rule

Source	Protocol	Type
0.0.0.0/0	ICMP	Echo Request

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2. Telnet in OCI

What is Telnet?

The telnet command is used to check connectivity to a specific TCP port.

It verifies whether a particular service is reachable.

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OCI Example of Telnet

Suppose:

• Web Server IP: 10.0.1.20
• SSH Port: 22

Command:

telnet 10.0.1.20 22

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Successful Telnet Output

Trying 10.0.1.20...
Connected to 10.0.1.20.
Escape character is '^]'.
SSH-2.0-OpenSSH_8.0

Meaning

• Server is reachable
• TCP Port 22 is open
• SSH service is running

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Failed Telnet Output

Connection refused

OR

Unable to connect to remote host

Possible Reasons

• Port blocked in Security List/NSG
• Service is not running
• OS firewall blocks the port
• Wrong routing configuration

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OCI Requirement for Telnet

TCP Port must be allowed in:

• Security List
• NSG
• OS Firewall

Example Rule

Source	Protocol	Port
0.0.0.0/0	TCP	22

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Real OCI Troubleshooting Scenario

Scenario

You are unable to SSH into an OCI instance.

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Step 1: Check Ping

ping 10.0.1.20

If Ping Fails

Possible Issues:

• Network issue
• Route Table issue
• ICMP blocked

If Ping Succeeds

Meaning:

• Network path is working correctly

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Step 2: Check Telnet on Port 22

telnet 10.0.1.20 22

If Telnet Fails

Possible Issues:

• SSH Port blocked
• SSH Service stopped
• Firewall issue

If Telnet Succeeds

Meaning:

• Port is accessible
• SSH service is reachable

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Key Difference Summary

Ping	Telnet
Checks server reachability	Checks service/port reachability
Uses ICMP	Uses TCP
No port required	Port required
Can work even if service is down	Requires service to be running
Used for network troubleshooting	Used for application/service troubleshooting

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Simple Real-Life Analogy

Tool	Analogy
Ping	Checking whether the house exists
Telnet	Checking whether a specific room door is open

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Conclusion

In OCI:

• Ping is used to verify basic network connectivity.
• Telnet is used to verify specific service port accessibility.

Both tools are very useful for troubleshooting:

• Security Lists
• NSGs
• Route Tables
• Gateways
• Firewalls
• Service Availability