Icmp Echo Quote by Wendell Odom
““First, to understand why the standard and extended ping results have different effects, consider first the standard ping 172.16.1.51 command on R1, as shown previously in Figure 23-7. As a standard ping command, R1 used its LAN interface IP address (172.16.1.1) as the source of the ICMP Echo. So, when the host (A) sent back its ICMP echo reply, host A considered the destination of 172.16.1.1 as being on the same subnet. Host A’s ICMP echo reply message, sent back to 172.16.1.1, would work even if host A did not have a default router setting at all! In comparison, Figure 23-8 shows the difference when using an extended ping on Router R1. An extended ping from local Router R1, using R1’s S0/0/0 IP address of 172.16.4.1 as the source of the ICMP echo request, means that host A’s ICMP echo reply will flow to an address in another subnet, which makes host A use its default router setting.””
About This Quote
Source Book: CCNA Routing and Switching Study Guide, Wendell Odom, 2020
The source IP used in a ping determines how the reply is routed; using a LAN interface IP keeps the reply on the same subnet, while using a different interface IP forces the reply to go through the default gateway.
In simple terms: Ping source IP decides reply path.
Key Takeaway
Choose appropriate source IP for testing.
Themes
networking
troubleshooting
ICMP
routing
subnetting
Mood
technical
analytical
Type
educational
technical
When to use this quote
- network diagnostics
- router configuration
- subnet verification
- troubleshooting labs
- educational labs
Key Concepts
IP addressing
default gateway
packet flow
Questions to Reflect On
- How does source IP affect reply routing?
- When would you prefer extended ping over standard ping?
A Different Perspective
If the default router is misconfigured, extended ping may still fail despite correct source IP.
