IPv4 and Subnetting

Network Layer (OSI Layer 3)

• Provides services for end devices to exchange data across networks.

Principle Network Layer Communication Protocols

• IP version 4 (IPv4).
• IP version 6 (IPv6).

Other Network Layer Protocols

• Open Shortest Path First (OSPF): a type of routing protocol.
• Internet Control Message Protocol (ICMP): a type of messaging protocol

Basic Operations of Network Layer

• Addressing end devices: each end device needs a unique IP address for network identification.
• Encapsulation: the network layer puts the transport layer PDU into a packet, adding IP header info like source and destination IP addresses, all done by the source of the IP packet.
• Routing: the network layer directs packets to a destination host, potentially across many routers, and requires router processing to travel across networks.
  • The role of a Router is to choose the best path and direct packets to the destination host using routing.
  • Hop: each router a packet passes through to reach its destination.
• De-encapsulation: at the destination host, if the packet's destination IP address matches the host's own IP address, the IP header is removed, then the resulting Layer 4 PDU is passed to the appropriate transport layer service, and this is done by the destination host.

IP

• Designed as a protocol with low overhead.
• Provides only the functions that are necessary to deliver a packet from a source to a destination over an interconnected system of networks

3 Basic Characteristics of IP

• Connectionless: no connection with the destination established before sending data packets.
• Best Effort: IP is inherently unreliable because packet delivery is not guaranteed.
• Media Independent: operation is independent of the medium carrying the data.

Fragmentation

• Happens when an IPv4 packet is split up when forwarded it from one medium to another with a smaller MTU.
• Causes latency
• IPv6 cannot be fragmented by the router.

IPv4

• IPv4 packet header makes sure the packet gets to its next stop on the way to where it needs to go.
• IPv4 packet header has fields of important info about the packet.
• These fields have binary numbers which are examined by the Layer 3 process.

Significant Fields in the IPv4 Header

• Version: a 4-bit binary value set to 0100, identifying the is an IPv4 packet.
• Differentiated Services or DiffServ (DS): an 8-bit field to set the packet's priority, where the six most important bits are the differentiated services code point (DSCP) bits, and the last 2 bits are the explicit congestion notification (ECN) bits.
• Time to Live (TTL): an 8-bit binary value that limits a packet's lifetime; the source device sets an initial value, each router decreases it by one, and if it hits zero, the router discards the packet and sends an ICMP Time Exceeded message to the source IP address, and routers must recalculate the Header Checksum because of this.
• Protocol: an 8-bit field identifying the next level protocol, with values like ICMP (1), TCP (6), and UDP (17), that helps the network layer pass data to the appropriate upper-layer protocol.
• Header Checksum: used to detect corruption in the IPv4 header.
• Source IPv4 Address: uses a 32-bit binary value that's the source IPv4 address of the packet, always a unicast address.
• Destination IPv4 Address: uses a 32-bit binary value that's the destination IPv4 address of the packet, and that destination can be unicast, multicast, or broadcast.

Source and Destination IP Address

• The two most referenced fields.
• Identifies packet origin and destination.
• These addresses do not change while traveling from the source to the destination.

Limitations of IPv4

• IPv4 address depletion: IPv4 has a limited number of unique public addresses available, even though it has about 4 billion addresses.
• Lack of end-to-end connectivity: Network Address Translation (NAT) lets multiple devices share a single public IPv4 address, hiding internal host addresses, which can be troublesome for tech that need end-to-end connectivity.
• Increased network complexity: NAT, a temporary solution for IPv4 has made networks more complex, causing latency.

Improvements that IPv6 Provides

• Increased address space: IPv6 uses 128-bit hierarchical addressing, where IPv4 uses 32 bits.
• Improved packet handling: IPv6 header is simpler with fewer fields.
• Eliminates the need for NAT: with many public IPv6 addresses, NAT between private and public IPv4s are not needed.

IPv4 and IPv6 Address Spaces

• 32-bit IPv4: 4,294,967,296 unique addresses.
• IPv6: 340 undecillion addresses.

IPv6 Packet Header Fields

• Version: a 4-bit binary value that's 0110, showing its an IPv6 packet.
• Traffic Class: this 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field.
• Flow Label: the 20-bit field suggests that all packets with the same flow label receive the same type of handling by routers.
• Payload Length: this 16-bit field shows the length of the IPv6 packet's data or payload but does not include its fixed 40-byte header.
• Next Header: this 8-bit field is equivalent to the IPv4 Protocol field and indicates the data payload type, letting the network layer pass data to the correct upper-layer protocol.
• Hop Limit: this 8-bit field takes the place of the IPv4 TTL field, decreasing by 1 for each router the packet passes. The packet is discarded if the counter reaches 0 and an ICMPv6 Time Exceeded message is forwarded to the sending host.
• Source IPv6 Address: the 128-bit field identifies the IPv6 address of the sending host.
• Destination IPv6 Address: the 128-bit field identifies the IPv6 address of the receiving host.

Sending Packets (Hosts)

• Itself: pinging localhost (127.0.0.1, or ::1 for IPv6) tests the host's TCP/IP stack.
• Local host: a destination on the same network as the sending host, sharing the same network address.
• Remote host: a destination on a different network without a shared network address.

Determining Destination IP Address

• In IPv4: the source device uses its own subnet mask along with its IPv4 address and the destination IPv4 address.
• In IPv6: local routers tells all devices the local network address (prefix).

Default Gateway

• A router that can route traffic to other networks.
• Usually has, a local IP address in the same range as other hosts, accepts data into the local network,. forwards data out, and routes traffic to other networks.

Route Printing

• route print or netstat -r displays the host routing table.
• Displays interface list, IPv4 Route Table with known IPv4 routes, and IPv6 Route Table of known IPv6 routes.

Routing Table Entries

• Directly-connected Networks: active router interfaces, interfaces create a directly connected route when configured with an IP address and activated, and connect to differnet network segments.
• Remote Networks: route entries connect to other routers, can be explicitly configured by an administrator, exchange route information with by a dynamic routing protocol.
• Default Route: most routers include a default route entry as a gateway of last resort,. and is used when there is no appropriate match in the IP routing table

How Routers Learn

• Routers learn about remote networks in two ways.
• Manually: remote networks are manually entered into the route table via static routes.
• Dynamically: through dynamic routing protocols.

Static Routes

• Route entries are configured manually. They include the remote network address and the IP address of the next hop router.

Characteristics of Static Routing

• Requires manual configuration.
• Requires reconfiguration by the administrator if there is a change in the topology and the route is no longer viable.
• Best for small networks or networks with few or no redundant links.

Dynamic Routing

• Allows routers to automatically learn about remote networks, including a default route.
• Ex: OSPF and Enhanced Interior Gateway Routing Protocol (EIGRP).
  • Discover remote networks.
  • Maintain up-to-date routing information. _ Choose the best path to destination networks. Attempt to find the best path if current path is no longer available.
• show ip route (privileged EXEC mode) views the IPv4 routing table.

Common Route Sources

• L - directly connected local interface IP address
• C - directly connected network
• S - static route was manually configured by an administrator
• O - OSPF
• D - EIGRP

Network Layer Characteristics (Summary)

• The network layer (OSI Layer 3) provides services to allow end devices to exchange data across networks. IPv4 and IPv6 are the principle network layer communication protocols. The network layer also includes the routing protocol OSPF and messaging protocols such as ICMP. Network layer protocols perform four basic operations: addressing end devices, encapsulation, routing, and de-encapsulation. IPv4 and IPv6 specify the packet structure and processing used to carry the data from one host to another host.
• IP encapsulates the transport layer segment by adding an IP header, which is used to deliver the packet to the destination host. The IP header is examined by Layer 3 devices as it travels across a network to its destination.
• The characteristics of IP are that it is connectionless, best effort, and media independent. IP is connectionless, meaning that no dedicated end-to-end connection is created by IP before data is sent, nor does it guarantee that all packets are delivered. IP operates independently of the media that carry the data at lower layers of the protocol stack.

IPv4 Packet (Summary)

• An IPv4 packet header consists of fields containing information about the packet. These fields contain binary numbers which are examined by the Layer 3 process. The binary values of each field identify various settings of the IP packet.
• Significant fields in the IPv4 packet header include: version, DS, header checksum, TTL, protocol, and the source and destination IPv4 addresses.

IPv6 Packet (Summary)

• IPv6 increases the available address space, improves packet handling, and eliminates the need for NAT. The fields in the IPv6 packet header include: version, traffic class, flow label, payload length, next header, hop limit, and the source and destination IPv6 addresses. IPv6 is designed to overcome the limitations of IPv4 including IPv4 address depletion, lack of end-to-end connectivity, and increased network.

Addresses (Summary)

• A host can send a packet to itself, another local host, and a remote host. In IPv4, the source device uses its own subnet mask along with its own IPv4 address and the destination IPv4 address to determine whether the destination host is on the same network. In IPv6, the local router advertises the local network address (prefix) to all devices on the network, to make this determination.
• The default gateway is the network device (i.e., router) that can route traffic to other networks. A host routing table will typically include a default gateway. In IPv4, the host receives the IPv4 address of the default gateway either: dynamically via DHCP, or it is configured manually
• In IPv6, the router advertises the default gateway address, or the host can be configured manually.

Intro to Routing (Summary)

• Routers examine the packet's destination IP address and search its routing table The routing table contains a list of all known network addresses and where to forward the packet. These entries are known as route entries or routes and the the router will forward using the best (longest) matching route.
• The routing table of a router stores three types of route entries: directly connected networks, remote networks, and a default route. Routers learn about remote networks manually, or dynamically using a dynamic routing protocol.
• Static routes are manually configured and include the remote network address and the IP address of the next hop router. OSPF and EIGRP are two dynamic routing protocols. The show ip route privileged EXEC mode command is used to view the IPv4 routing table on a Cisco IOS router.

Addresses Assigned to a Device on an Ethernet LAN

• Physical address (MAC address): for NIC to NIC comms on the same Ethernet network.
• Logical address (IP address): sends the packet from source to destination device, which may be on same IP or remote network.
• When the destination IP address is on a remote network, the destination MAC address will be the address of the host default gateway.

Layer 2 Frame addresses

• Destination MAC address of the destination device on the same local network segment (or default gateway if on another network).
• Source MAC address of the Ethernet NIC on the source host.

Address Resolution Protocol (ARP)

• Used to determine a destination MAC address of a local device when it knows its IPv4 address.
• Two basic functions is resolving IPv4 addresses to MAC addresses, and maintain a table of IPv4 to MAC address mappings.

ARP table or ARP cache

• Temporary RAM storage to find the MAC address that maps to the IPv4 address.
• If the packet and destination IPv4 address are on the same network, the device will search the ARP table for the destination IPv4 address.
• If the packet and destination IPv4 address are NOT on the same network, the device will search the ARP table for the IPv4 address of the default gateway.

ARP Request

• Sent when a device must determine the MAC address linked to an IPv4 address, and has no entry for the IPv4 address in its ARP table.

ARP Messages

• Encapsulated directly within an Ethernet frame, there is no IPv4 header, the ARP request is encapsulated in an Ethernet frame using the following header information:
  • Destination MAC address: broadcast address FF-FF-FF-FF-FF-FF which requires every Ethernet NIC on the LAN to accept/process.
  • Source MAC address: MAC address of the sender of its request.
  • Type: ARP messages contain the "type field of 0x806" which tell receiving NICs that data needs to be passed to the ARP process
• To display the ARP table:
  • show ip arp - a cisco router
  • arp -a – windows 10

IPv6 Neighbor Discovery (ND or NDP)

• Provides address resolution, router discovery, and IPv6 redirection services using ICMPv6.
• ICMPv6 ND uses five ICMPv6 messages to perform these services:
  • Neighbor solicitation: messages are sent out using special Ethernet and IPv6 multicast addresses to determine whether the Neighbor Solicitation message is for itself without having to send it to the operating system.
  • Neighbor advertisement.
  • Router solicitation.
  • Router advertisement mesages.
  • Redirect message for better next hop selection.
• Neighbor solicitation and neighbor advertisement are used for device messaging (such as address resolution) and includes computers and routers.
• Router solicitation and router advertisement are used for messaging between devices and routers.

Summary of Networking

• Layer 2 physical addresses (MAC addresses) are used to deliver data link frame from one NIC to another NIC on the same network, and if the destination IP address is on the same network, destination MAC address will be that of the destination device. When the destination IP address is on a remote network, the destination MAC address will be the address of the host default gateway (i.e., the router interface). Along each link in a path, an IP packet is encapsulated in a frame and if the next-hop device

Explore IPv4 addressing, subnetting, and routing. Learn how devices determine network locations and how routers forward packets. Understand default gateways and routing tables.