Design and Planning

Ubuntu

IP Information

Routing tables determine how a packet reaches its destination based on the subnet. If the destination IP address is on the same subnet as the source, the packet is delivered directly. If it is on a different subnet, the packet is sent to the default gateway, which forwards it to the appropriate network beyond the LAN.

Linux

IP Information

When using the ping comand on traffic traveling in the same subnet, the default gateway is not required because delivery occurs directly between devices. A gateway entry is unnecessary in this case since both devices share the same network ID.

Tecnical Development

In the first ping test, Ubuntu delivered the packet directly to its destination because both IP addresses were on the same subnet. No router interaction was required.

In the second test, the packet had a destination outside of the local subnet thus the packet could not be delivered directly. The packet was therefore forwarded to the default gateway and passed through a router before reaching its destination network.

• Observation: the packet that left the local subnet took significantly longer to reach its destination as opposed to the packet that stayed on the local subnet.

Below, the IPv4 on the VM is circled in red. Because the IP is private, there could be another device on a separate private network with the same IPv4. Because the networks are private they would not conflict. A device may appear to have two IP addresses because it uses a private address internally and a public address externally. The private IP is assigned by the router and used within the LAN while the public IP is used to identify the device on a broader network/internet.

RFC1918 Address Ranges

• 24-bit Block: 10.0.0.0 - 10.255.255.255
• 20-bit Block: 172.16.0.0 - 172.31.255.255

• 16-bit Block: 192.168.0.0 - 192.168.255.255

• The IP of the Ubuntu VM falls into the first range

One of the VMs IP addresses is strictly used for communication outside of the network, while the other VM is used privatly within the network.

Partner Ping

Both partner devices share the same public IP address. Pinging the public IP is useless because the device is merely pinging itself. Failure does not indicate the device does not exist; rather, it suggests that the router cannot determine which device should receive the packet. Private IPs allow the router to identify where on the specific network the packet is supposed to go.

Testing and Evaluation

The original topology consisted of two PCs, two switches, and one router. After configuration, devices on the left side of the topology were able to communicate successfully. Configuring an additional router allowed Switch 1 and the router to exchange traffic properly. Once both router interfaces were active and configured, all connected devices were able to communicate across networks.

• Set up after configuring the ports properly

Private IPv4 addresses are reused across millions of networks to conserve the limited IPv4 address space. If every device required a globally unique public address, IPv4 exhaustion would have occurred much earlier. Private addresses are not routed on the public internet because they are not globally unique and would create addressing conflicts. Some devices effectively operate with two identifiers: a private IP for internal communication and a public IP for external communication. This significantly influences WAN design by simplifying address management and reducing public IP requirements.

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As the packet moves across the devices, the destination and source remain constant.

The switch does never modifys the IP address because it oparates in the 2nd layer of the OSI Model. IPs exist and function on the 3rd layer of the OSI Model. Additionally, the router changes the MAC addresses of the packets as they are in the internal network. The source IP is constant between PC0 and PC1 because the sender/soure did not change as it traveled to its destination. Next hop referes to the next step or device the packet will reach when moving to its destination. The default gateway is necessary as the packet is moving through the router and into a different network.

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The routing table indicates that the device is directly connected to the enp0s1 interface and uses 10.12.16.1 as its gateway. Traffic destined for external networks is forwarded to this default gateway.

8.8.8.8

• Initially, the traceroute displayed *** meaning that the packet did not recive a response in the given timeout. After powering off the Ubuntu VM and reloading it, the traceroute 8.8.8.8 command worked correctly.

In traceroute to 8.8.8.8, the first hop is the gateway IP address, allowing it to exit the local network. After the second hop, the addresses are no longer private IPs. The packet reaches its destination in a total of 10 hops.

Again, after restarting the VM, the command worked properly.

When tracing to google.com, the first hop remained the same gateway IP. However, the total hop count was significantly higher. Both traces shared identical early hops before diverging at a later ISP routing point.

The most likey next hop would be to 10.12.16.1 still using the enp0s1 interface. This aligns with the traceroute results, having the same gateway IP for the first step.

The ip route get command displays only the next-hop routing choice, whereas traceroute shows the complete path the packet travels, highlight the network layer, layer 3.

Time to Live - TTL

Routers reduce a packet’s Time to Live (TTL) value at each hop to prevent the network from becoming overloaded with packets that never reach their destination. This protocol ensures that packets are discarded after a set limit rather than circulating indefinitely. Traceroute takes advantage of TTL to identify and display the route a packet follows to its destination.

Determining Packet Paths

The initial hop in a packet’s path reveals whether the destination is on the same network as the source or requires leaving the LAN. If the packet is sent to the default gateway, it indicates that the destination is on a different network. Private IP addresses typically appear at the beginning of the route because the packet is still traveling within its local network, where devices communicate using private rather than public addresses. It is also possible for two different destinations to share the same first few hops, since both packets may follow the same route out of the LAN before branching off across separate networks. The key distinction between a traceroute and a routing-table lookup lies in the level of detail: traceroute displays the full path a packet may take, while the routing table only shows the immediate next-hop decision rather than the complete route.

Reflection

This project reinforced how routing tables, default gateways, and subnetting work together to direct network traffic. It demonstrated the difference between communication within the same LAN and communication across separate networks, highlighting when router involvement is required. The testing process clarified how private and public IP addresses function, as well as why private IPv4 ranges are reused and not routed on the public internet. Observations from ping and traceroute commands showed how hop counts, TTL, and next-hop decisions reveal the path a packet takes. Overall, the activities strengthened understanding of Layer 2 and Layer 3 responsibilities and how devices cooperate to move data efficiently across networks.