Network Layer Fundamentals

Network Layer Overview

• It is the third layer in the OSI model
• Responsible for host-to-host delivery
• It gets packets from the source to the destination, via intermediate nodes
• Source node accepts and encapsulates data into a datagram for the Data Link layer
• Destination node decapsulates the datagram, extracting the packet and delivering it to the Transport Layer
• The layer defines logical addresses used at layer-3.
• It directs paths to destination components based on network numbers of logical addresses.
• Connects different data link layer types together like Ethernet, FDDI, Serial, and Token Ring

Network Services

• Internetworking refers to the logical connection of heterogeneous physical networks
• It makes them look like a single network to the upper transport and application layers
• An internetwork can consist of networks, such as 4 LANs and 1 WAN
• Switch or router (Si) defines the interface (fi)
• Data sent from point A to D passes through 3 links
• The source and destination are referred to as hosts
• A host or router can be called a "hop"

Packetization and Fragmentation

• Packetizing encapsulates packets from upper-layer protocols, creating new packets.
• This is handled by the IP protocol, which is part of the Internet model
• Fragmentation allows datagrams to travel through different networks.
• Each router decapsulates the IP datagram from the received frame and re-encapsulates it into another frame after processing.
• Size and format of received and departing frames depends on the protocol of physical network that the frame has just arrived, or is departing.
• Routing carries pieces of information from one network to another.

Addressing

• Addressing is how data knows where to go
• IP addresses are used to uniquely identify a packet's source and destination
• IP addressing identifies any host or router interface connected to a TCP/IP network
• IP has different versions: IPv4, IPv5, and IPv6
• Internet Protocol (IP)
  • IP is a connectionless protocol
  • It provides routing of datagrams in a best-effort approach
  • The IP datagram is a combination of bytes that includes an IP header prefixing data from the transport layer, or higher
  • IP addressing identifies end stations involved in the transport of datagrams

Types of IP Addresses

• Reserved Addresses: Cannot be assigned to a network component
• Network ID: The network number assigned to a segment or VLAN
• Host ID: The host component of an IP address
• Directed Broadcast: The broadcast address of a network ID, which can be routed by routers
• Local Broadcast: The "all hosts" address, 255.255.255.255, is for the same segment and is not routed by routers
• Loopback: The internal address of a device used for testing functions
• Autoconfigured: Addresses assigned automatically to a network component, such as DHCP
• Public: Used to access devices across the Internet
• Private: Used to access devices in a local network, not for public networks
• A 32-bit address uniquely defines a device’s connection

IPv4 Addressing

• An IPv4 address has two parts: network and host
• Network Address identifies the network part
• Host Address is assigned to end devices
• IPv4 provides "Address Space"
• A total number of addresses used by the protocol equals 2n, for an n-bit address
• IPv4 has an address space of 232, which equals 4,294,967,296
• IPv4 uses two notations:
  • Dotted Binary Notation: Employs binary bits e.g., 00001010.00001010.00010100.11101001
  • Decimal Notation: Uses decimal numbers e.g., 192.168.10.4
• IPv4 includes five different classes

IPv4 Address Classes

• Classes A, B, and C are primary addresses for data traffic
• Class D is used for multicasting
• Class E is reserved
• Number of host bits in the IP address classes determines how many hosts can be created
• Number of host addresses = 2n - 2
• Class A, B, and C have private and public address ranges, for use in internal networks (intranets) and public TCP/IP networks

Internet Governance

• Internet Corporation for Assigned Names and Numbers (ICANN) governs domains and addresses
  • It is a not-for-profit, public-benefit corporation
  • Dedicated to keeping the Internet secure, stable, and interoperable
  • It promotes competition and develops policy on identifiers like DNS names and Autonomous System numbers
• Internet Assigned Numbers Authority (IANA) maintains a central repository
  • Verifies and updates Top Level Domain (TLD) information
  • Distributes Internet numbers to regions for use
  • It manages IP addresses, ports, protocols, and other essential numbers

IP Address Problems and Subnetting

• Address efficiency can be affected by IP address wastage or shortage
• Subnetting takes higher-order host bits in a network number to create more networks
• It breaks down or partitions networks into subnets
• Subnet Mask
  • Identifies bits used to represent the network/subnet portion of an IP address
  • Differentiates between the network address, the host addresses, and the directed broadcast address
  • Its length is 32 bits long
  • 1's represent the Network component
  • 0's represent the Host components
• Subnets are created using subnet masks and by borrowing bits from the host portion of the IP address
• The network portion of the IP address combined with the new subnet bits defines new subnet

Subnetting Details

• Each net address loses two addresses
  • Network Address: One address is reserved for the network, e.g., 172.16.0.0 /16
  • Broadcast Address: One address is reserved for the hosts, e.g., 172.16.255.255
• Create new subnets by borrowing bits from the host portion of the IP address

Classful Addressing Issues

• With classful addressing, too much waste of host addresses occurs
• A potential solution involves Classless Inter-Domain Routing
• Classful addressing divides the IPv4 address space (0.0.0.0-255.255.255.255) into 5 classes (A, B, C, D, E)
• The first few bits indicate the class of an address

Number of Subnets and Hosts

• Number of subnets created = 2x (x is the number of bits borrowed from the host bits)
• Number of hosts/subnet = 2y-x (y is Number of host bits for the class of the network)

Subnet Mask

• The subnet mask is often expressed as /24
• The number of bits to borrow, x, is 2
• The number of bits left in the host portion, y, is 6
• The subnet mask of the new subnet would be /26
• The network address is 192.168.10.0/26
• The address Space = 192.168.10.0 - 192.168.10.63

Classless Inter-Domain Routing (CIDR)

• CIDR addresses the limitations of classful addressing, inefficient allocation and routing table scalability
• CIDR allows for variable-length subnet masks (VLSM) and supernetting
• Variable-length subnet masks (VLSM) allows the application of different subnet masks to the same class address space, called "Subnet-of-Subnets"

IPv4 Datagram

• Version indicates the IP version
• IHL indicates header length in 32-bit words
• Differentiated Services is intended to distinguish between classes of service but is mostly ignored by current routers
• Total length indicates the datagram's total length, up to 65,536 bytes
• Identification helps the destination host determine the datagram to which a fragment belongs
• DF (Don’t Fragment) tells routers not to fragment the packet
• MF indicates that all fragments except the last one have a bit set
• Fragment offset indicates where the fragment belongs in the current packet
• Time To Live limits packet lifetimes
• Protocol indicates the transport process to give the packet to
• Header checksum performs an arithmetic checksum
• Source address and Destination address indicate the IP addresses of the interfaces
• Options provides an escape to allow subsequent versions of the protocol

IPv6 Addressing

• IPv6, also called IPng, expands the possible number of users on the Internet because of Rapid Growth of Appliances
• IPv6 uses a 128-bit address and uses hexadecimal colon notation
• IPv6 has many zeros which leads to needs to omit or represent

IPv6 Addressing Details

• Can abbreviate by omitting leading zeros and not trailing
• IPv6 has many types and it split by addresses
  • Global unicast
    • These addresses are routable, starting with "2001:" or group.2000::/3.
    • Its the equivalent of IPv4 public addresses
  • Unicast Address
    • It single defines a device
    • Two types ( Geographic-based and Provider-based)
  • Multicast address has a group of hosts. 7→ FF00::/8
    • Each packet is delivered to member.
  • Local address (FE80::/10)

IPv6 Formats and Classes

• The other types of IPv6 address FE80::/10
• It used from Organizations to use IPv6 protocol without being connected to the Internet.
• There are analogous Private addresses of IPv4.
• Types:
  • Link Local addresses
  • Site Local addresses

IPv6 Address Types

• Anycast Address is for packet to all members
• Reserved Addresses start is prefix type with 8 0's
  • The Unspecified Address is to use to find own node
  • Addresses loop back its for test

IPv6 Header and Details

• The IPv6 Header includes:
  • Version (4 bits) is always 6
  • Traffic class (4 bits) distinguishes packets for delivery requirements
  • Flow label (24 bits) provides handling for a particular data flow
  • Payload length (16 bits) indicates datagram length, excluding the 40-byte base header
  • Next header (8 bits) defines the header that follows either the headers or optional extension headers or such as UDP/TCP
  • Hop limit (8 bits) is the same as IPv4’s Time To Live field
  • Source address and Destination address (16 bytes each)

Routing Protocols

• Packet-switching network complex is routing
• Routing protocols provide route management. Provides
  • Route selection, sharing route status with neighbor routers
  • Calculating alternative routes if the main route is down
  • To accept packets from a source station and deliver them to a destination station

Routing Details

• Routing is conveying packets one network to another
• It needs to Establish and Maps
• Moving packets with Interfaces
• Packets may have has path, they store in routes
  • There are decisions that uses Topography
  • such delay

Forwarding and Routing Decisions

• A router establishes which interface a packet is sent to, via routing process
• Forwarding decisions involves
  • Destination address requirements
  • Service such packet filter
• Routers use routing for destinations
• Routing has 2 type
  • Static Routing (Non-Adaptive Routing).
  • Adaptive or Dynamic Routing

Routing Types

• Non-Adaptive Routing(Static routing)
  • Estimates traffic
  • Does useful is traffic
  • Path is set
    • Manual
  • Troubleshooting.
  • Uses Static is Poor internet stategy
• Adaptive Routing(Routing)
  • reflect Changes to traffic
  • such topology

Dynamic Routing Algorithms

• Dynamic routing algorithms ask Where do they get their information and routes
  • From agents
• Ask when the metrics
• Such is second Load of times

Issues with Protocols

• Path Determination has procedures
  • Protocol uses to pathing
• Metric has such reliability best with count
  • Numbers are bad
  • Convergence that view with
  • Takes time
• Load balancing can be used with packet router
  • There are Distances Protocols
  • Link State protocols

Distance Vector Protocols

• Decisions is of distance
  • Numbers measured is hops
  • router through has hops
• Send routers in networks
• Router RIP is uses one
• Router IGRP

Link-State Routing

• Also called shortest-path-first protocols
• Complete picture to router
• Separate tables
  • Uses details to connect
• Topology to network
• Actual table
• Routing connects the links
  • OSPF -IS-IS
  • EIGRP