Layer 3 (Network)

How wer’re going to forward traffic using logical addresses. iP address IPV4 or IPV6.

Logical addressing - IPV4 and IPV6 - AppleTalk, IPX are old versions.

IP:

IPV4 - ex. 10.0.0.38/24

Layer 3 switching (Routing) -

packet switching - Data is devided into packets and then forwarded on. like mailing a letter through different mail facilities.

circut switching - Dedicated communication link is established between the two deivices. like the old school phone lines. a dedicated and constant communication path is maintained for the duration of a converstion.

message switching: data is divided into messaged which may be stored then forwarded.

most networks are using packet switching. home networks, and small networks.

Route discovery and selection: how to know which wya to let the traffic to go. Manually configured as a static route or dynamically through a routing protocol using rouging tables isign rougint table. RIP OSPF and EIGRP

How to know which way to go can be done by routing protocols similar to GPS

Connection services: Augment layer 2 services to improve reliablity

flow control

packet reordering - encures that all data reaches the reciver correctly at the end destination.

ICMP - Sends error messages and operational information to an IP destination.

ping , tracert

bandwidth usage

Router

Multi-layer switcher

🌐 Layer 3 — Network Layer (OSI Model)

Purpose: Determines how traffic is forwarded using logical addressing (IP). Responsible for routing, packet forwarding, and path selection across networks.

🧠 Key Concepts

Uses logical addresses (IPV4, IPV6) to identify devices across networks.
Supports packet delivery between networks (unlike Layer 2 which is limited to a local segment).

🧾 Logical Addressing

IPV4 Example: 10.0.0.38/24
- /24 indicates the subnet mask (255.255.255.0)
IPV6: Longer addresses, written in hex (e.g., fe80::1)
Legacy protocols: AppleTalk, IPX — no longer in use

🔄 Layer 3 Switching (Routing Types)

| Type | Description | Analogy | | --- | --- | --- | | Packet Switching | Data is split into packets and forwarded independently | Like mailing separate letters | | Circuit Switching | A dedicated communication path is created for the whole session | Like an old-school phone call | | Message Switching | Whole messages are stored and forwarded later | Like handing off a written message at each stop |

🧠 Most networks today use packet switching (including your home router).

🧭 Route Discovery & Path Selection

Goal: Decide the best path to reach a destination network

Two ways to configure routes:

✅ Static Routing – Manually set paths
✅ Dynamic Routing – Uses protocols to auto-discover best paths

Routing Protocol Examples:

| Protocol | Type | Description | | --- | --- | --- | | RIP | Distance-vector | Based on hop count, simple but outdated | | OSPF | Link-state | Calculates fastest route using bandwidth | | EIGRP | Hybrid | Combines features of both methods |

Similar to a GPS choosing the fastest or shortest route based on live data.

🔗 Layer 3 Services

Connection Services: Improve transmission reliability
- Flow control
- Packet reordering (ensures correct sequence)
ICMP (Internet Control Message Protocol):
- Used for diagnostics (e.g., ping, tracert)
- Sends error and status messages (e.g., destination unreachable)

📈 Bandwidth Usage Concepts

QoS (Quality of Service) often works at Layer 3 to prioritize critical traffic (VoIP, video).
Load balancing and routing metrics influence path decisions and congestion handling.

🛠️ Layer 3 Devices

| Device | Role | | --- | --- | | Router | Connects different networks, makes forwarding decisions | | Multilayer Switch | Acts as both Layer 2 (switch) and Layer 3 (router) | | Firewall (Layer 3 capable) | Can filter based on IP and ports | | Core Routers / Edge Routers | Used in large enterprise and ISP networks |

📌 Summary

| Function | Layer 3 Role | | --- | --- | | Addressing | Assigns and uses IP addresses | | Routing | Forwards packets based on routing tables | | Diagnostics | Uses ICMP to test and report | | Delivery | Ensures packets reach the destination subnet | | Connection Enhancements | Works with Layer 2 to increase reliability |