How Does Data Travel from Client to Server? A Deep Dive into Network Layers

01 Client-side Data Encapsulation

Java client code (illustrated) shows how the application layer specifies the server domain or IP, port number, and payload, then passes them to the transport layer.

The transport layer treats the payload as its data and prefixes it with a source port (dynamically assigned by the OS in the 49152‑65535 range) and a destination port (fixed in the Java code).

The network layer adds source and destination IP addresses; if a domain name is used, it is resolved via DNS following the sequence: browser cache → operating‑system cache → hosts file → DNS server.

The data‑link layer adds the source MAC address (the client’s NIC) and the destination MAC address of the next‑hop device, because the client cannot know the server’s MAC directly.

ARP broadcasts a request announcing its IP and MAC; hosts on the same subnet reply and update their ARP tables with IP‑to‑MAC mappings.

If the target IP is absent from the ARP table, the routing table (generated from /etc/sysconfig/network-scripts/ifcfg-eth0) is consulted. The table matches the destination IP with the most specific subnet mask to determine the next‑hop IP (gateway) and then looks up that IP’s MAC via ARP.

02 Switch Forwarding

The switch receives the frame, reads the destination MAC at the data‑link layer, consults its ARP table, and forwards the frame out the appropriate port (e.g., Fa2/1) toward the router.

03 Router Routing

The router parses the packet at the network layer, identifies the destination IP (e.g., 192.168.3.6), and consults its routing table to select the correct outgoing interface (e.g., FastEthernet0/1).

It then replaces the destination MAC with the MAC of the next hop (obtained from the router’s ARP table) and forwards the packet to the next switch.

04 Server-side Data Decapsulation

The server receives the frame, strips each layer in reverse order, and finally delivers the payload to the application program. After processing, the server sends a response packet following a similar path, which may differ because intermediate routing can change.

05 Extensions

Additional tools include DNS lookup to view the IP address associated with a domain and traceroute (e.g., tracert or traceroute) to count the number of routing nodes between the client and server.