Can Direct Connections in Different Network Segments Ping Each Other?

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Introduction:

When two devices are directly connected, they can ping each other if they are on the same network segments. However, if they are on different network segments, they may not be able to ping each other.

In this article, we will explain why serial and Ethernet direct connections can ping each other and why they may not be able to ping each other if they are on different network segments.

Serial Direct Connections:

Serial direct connections use the Point-to-Point Protocol (PPP) to establish a connection between two devices. PPP is a connection-oriented protocol that provides a reliable link between two devices.

When two devices are connected using PPP, they exchange their MAC addresses during the negotiation process. This allows them to identify each other and send packets directly to each other without the need for ARP.

Therefore, serial direct connections can ping each other even if they are on different network segments.

Ethernet Direct Connections:

Ethernet direct connections use the Ethernet protocol to establish a connection between two devices. Ethernet is a connectionless protocol that does not provide a reliable link between two devices.

When two devices are connected using Ethernet, they need to use ARP to resolve the MAC address of the destination device. ARP is a protocol that maps IP addresses to MAC addresses.

If two devices are on the same network segment, they can send ARP requests to each other directly. However, if two devices are on different network segments, they need to send ARP requests to the default gateway.

If the default gateway is on the same network segment as the destination device, the ARP request will be successful. However, if the default gateway is on a different network segment, the ARP request will not be successful.

Therefore, Ethernet direct connections may not be able to ping each other if they are on different network segments.

Conclusion:

In summary, serial and Ethernet direct connections can ping each other for different reasons. Serial direct connections use PPP to establish a connection, which allows them to identify each other and send packets directly to each other. Ethernet direct connections use ARP to resolve the MAC address of the destination device, which may not be successful if the destination device is on a different network segment.

ARP Process Simulation in Ether Connections:

I use Cisco Packet Tracer to simulate two scenarios:

Same segment:

R1 interface ip address is 3.3.3.3/24, R2 interface ip address is 3.3.3.4/24.

The ARP request is broadcast on the local network segment.

The device with the matching IP address responds to the ARP request with an ARP reply.

The device that sent the ARP request stores the MAC address of the destination device in its ARP cache.

different segments:

R1 interface ip is 3.3.3.3/24, R2 interface ip is 4.4.4.4/24

This is a packet trace log on R2, it tells us R2 receives a GRATUITOUS type ARP from R1, but as shown in the description, the ARP process drops the frame and doesn’t save the IP address.

GRATUITOUS type ARP is used to detect IP conflict. R2 can’t receive the broadcast-type ARP message from R1, so R2 doesn't know R1’s MAC address.