M5: Network Layer (3)

M5 CISCO Network Device: Explain how routers use network layer protocols and services to enable end-to-end connectivity.

Layer 3 Network

Moves packets between networks (end‑to‑end delivery). Uses logical addressing (IPv4/IPv6). Encapsulates transport‑layer data into packets. De‑encapsulates packets at the destination. It is responsible for getting data from one device to another across different networks. It is about global delivery.

3 Characteristics of IP

• Connectionless

• Best Effort

• Media Independent

Connectionless

When IP sends data, it does not establish a session, perform a handshake, or confirm the destination is ready. In short IP does not establish a connection before sending data.

Best Effort

IP does not guarantee delivery, order, integrity, or speed. it simply tries.

Media Independent

IP works over any physical medium: copper, fiber-optic, or wireless. It doesn’t care how the bits travel.

Maximum Transmission Unit

The largest packet size a network medium can carry. If a packet is too large, IP may need to fragment it.

Fragmentation

Splits a large IP packet into smaller pieces. Causes latency and processing overhead. Should be avoided when possible.

Layer 3 Operations

1. Addressing End Devices

2. Encapsulation

3. Routing

4. De-encapsulation

IPv4 Packet Header

Contains instructions for how the packet should be handled. Helps routers make forwarding decisions. Identifies the source and destination __ addresses. Routers use this information for forwarding.

Version

An IPv4 Header Field that tells the routers “This is an IPv4 packet.” 4 bit field.

Differentiated Services

An IPv4 Header Field that helps network prioritize traffic (voice over email). Formerly called ToS (Type of Service). 8 bits total

Time to Live

An IPv4 Header Field that limits how long a packet can stay in the network. 8 bit field. Decreases by 1 at every router hop. If it reaches to zero, router discards the packet. It’s purpose is to prevent packets from looping forever.

Protocol

An IPv4 Header Field that identifies the Layer 4 protocol carried inside the packet. 8 bit field. It’s purpose is to tell the network layer which upper-layer protocol should receive the data.

Header Checksum

An IPv4 Header Field that validates the IPv4 header. Detects corruption. It is recalculated at every hop. It’s purpose is to ensures header integrity.

Source IPv4 Address

An IPv4 Header Field that identifies the sender. 32 bit field. Always a unicast address. It’s purpose is so routers and the destination know where the packet originated.

Destination IPv4 Address

An IPv4 Header Field that identifies the intended recipient. 32 bit field. can be unicast, multicast, or broadcast. Usually do not change during transit except when NAT modifies them.

Three Major Limitations of IPv4

• Address Depletion

• Lack of End-to-End Connectivity

• Increased Network Complexity

IPv6 Improvements

• Increased Address Space (128 bit)

• Improved Packet Handling

• Eliminates the Need for NAT

IPv6 Header Improvements

• Simplified Header

• Fixed size (40 bytes)

• Some IPv4 fields removed

• Some fields renamed or repositions