Introduction to Network Engineering Notes

Internetwork: Refers to an arbitrary collection of networks interconnected to provide a host-to-host packet delivery service.
- Purpose of Network Layer:
- Solve the problem of host-to-host delivery.
- Facilitate routing packets over networks.
- Definition: An internetwork is often referred to as a "network of networks" as it comprises multiple smaller networks.

Link Layer Delivery: Involves hop-to-hop delivery, focusing on transferring packets between directly connected devices.
- Structures include physical and data link layers.
Network Layer Delivery:
- Involves end-to-end delivery, concerning the overall journey of packets from the source host to the destination.
- Comparison of Layers:
  - Link Layer: Responsible for data transfer between neighbor devices.
  - Network Layer: Responsible for the path across different networks.

Unique Addressing Scheme: Provides a method to identify all hosts in the internetwork.
Datagram Approach: Used for data delivery.
Connectionless Service: Does not establish a dedicated connection.
Unreliable Delivery:
- If a packet is lost, corrupted, or misdelivered, the network layer takes no corrective action.
- Characteristics include:
- Best-effort delivery with no guarantees.
- No flow control.
- No error control.

Structure:
- Header: 20-60 bytes
- Data: 20-65,536 bytes
Header Fields:
- VER: 4 bits (IP version)
- HLEN: 4 bits (Header Length)
- Service: 8 bits (Type of Service)
- Total Length: 16 bits (Total length of the datagram including header)
- Identification: 16 bits (Used for fragmentation)
- Flags: 3 bits (Indicate fragmentation possibilities)
- Fragmentation Offset: 13 bits (Position of fragment in datagram)
- Time to Live (TTL): 8 bits (Limits the lifespan of a datagram)
- Protocol: 8 bits (Upper-layer protocol encapsulated)
- Header Checksum: 16 bits (Error checking)
- Source/Destination IP Address: 32 bits each
- Options: 32 bits (Optional fields for routing, management, etc.)

Fixed and Optional Parts: The header consists of a fixed part of 20 bytes plus optional fields.
Fields Explanation:
- Version: Specifies the version of IP.
- Header Length (HLEN): Indicates the length of the header.
- Type of Service: Enables differentiated treatment of packets based on application needs.
- Total Length: Indicates the entire datagram size.
- Identification: Segments related to a datagram are tagged with the same identification number.
- Flags:
- Denote if fragmentation is allowed.
- Indicate fragment position (first, middle, last).
- Offset: Shows the relative position of fragments within the datagram.
- TTL: The maximum number of hops a datagram is allowed to traverse.
- Protocol: Codes for the upper-layer protocol used (e.g., TCP, UDP).
- Header Checksum: Helps in the detection of errors.
- Source/Destination Address: Identifies where the datagram originated and where it is going.
- Options: Fields to manage routing and protocols.

Definition: Maximum layer 3 packet size that can be forwarded out of an interface.
Basis: Determined by the data link layer protocol.
Default MTU: Commonly set to 1500 bytes for Ethernet.
Importance of MTU:
- If a packet exceeds the MTU size, it must be fragmented to fit within the allowable size.

Purpose: Required when the MTU of the interface is smaller than the packet size.
Mechanism: Splits the packet into smaller fragments for transmission.
Offset: Shows the position of fragments based on the overall datagram size in units of 8 bytes.

Address Classes:
- Class A: First byte starts with 0 (0-127).
- Class B: First byte starts with 10 (128-191).
- Class C: First byte starts with 110 (192-223).
- Class D: First byte starts with 1110 (224-239) (Multicast).
- Class E: First byte starts with 1111 (240-255) (Reserved for future use).

Block Ranges:
- 0.0.0.0 to 0.255.255.255: "This" Network
- 10.0.0.0 to 10.255.255.255: Private Use Networks
- 127.0.0.0 to 127.255.255.255: Loopback Address
- 192.168.x.x: Private Network Used
- 240.0.0.0 to 255.255.255.255: Reserved for future use.

Network Number: Represents a network in a 32-bit format, with the host portion zeros.
Broadcast Address: Used to address all hosts in a network, with the host portion composed entirely of ones.
Example: For IP Address 8.1.4.5:
- Network address: 8.0.0.0
- Broadcast address: 8.255.255.255.

Subnet Mask: Helps specify the network ID and host ID.
Bitwise Operation: Performing a bitwise AND between the IP address and subnet mask results in the network ID.

Definition: A node's internet address indicates its connection to a network, including both network (netid) and host parts (hostid).
Requirement for Routers: Devices such as routers, which connect multiple networks, need to have multiple addresses (one for each connected network).

Definition: Organizations that require IP addresses solely for internal communication without requiring direct internet access may use any network number.
RFC 1918: Defines networks that are reserved and may be used internally (Private Addressing) without valid registered IP addresses.
NAT (Network Address Translation): A method proposed for allowing hosts on private networks to communicate over the internet.

Advantages: Saves IP address space; provides internal identification without requiring external internet access.
Disadvantages: NAT must map private addresses to registered external addresses to enable internet communication.

Example: Shows how NAT translates private addresses to public addresses:
- Devices with private addresses (e.g., 172.18.3.x) interact via NAT router connected to the internet.

Function: DHCP dynamically assigns IP addresses to hosts.
Process:
- Host broadcasts a “DHCP discover” message.
- DHCP server replies with a “DHCP offer” message.
- Host sends a