Chapter 8: Network Layer Vocab

Itself

A host can ping itself by sending a packet to a special IPv4 address of 127.0.0.1 or an IPv6 address ::1, which is referred to as the loopback interface. Pinging the loopback interface tests the TCP/IP protocol stack on the host.

Addressing end devices

End devices must be configured with a unique IP address for identification on the network.

Default route

Like a host, most routers also include a default route entry, a gateway of last resort. The default route is used when there is no better (longer) match in the IP routing table. In the figure, the R1 IPv4 routing table would most likely include a default route to forward all packets to router R2

IPv4 Route Table

Lists all known IPv4 routes, including direct connections, local network, and local default routes.

IPv6 Route Table

Lists all known IPv6 routes, including direct connections, local network, and local default routes.

Interface List

Lists the Media Access Control (MAC) address and assigned interface number of every network-capable interface on the host, including Ethernet, Wi-Fi, and Bluetooth adapters.

Lack of end-to-end connectivity

Network Address Translation (NAT) is a technology commonly implemented within IPv4 networks. NAT provides a way for multiple devices to share a single public IPv4 address. However, because the public IPv4 address is shared, the IPv4 address of an internal network host is hidden. This can be problematic for technologies that require end-to-end connectivity.

Media Independent

Operation is independent of the medium (i.e., copper, fiber-optic, or wireless) carrying the data.

Manually

Remote networks are manually entered into the route table using static routes.

Dynamically

Remote routes are automatically learned using a dynamic routing protocol.

Improved packet handling

The IPv6 header has been simplified with fewer fields.

Encapsulation

The network layer encapsulates the protocol data unit (PDU) from the transport layer into a packet. The encapsulation process adds IP header information, such as the IP address of the source (sending) and destination (receiving) hosts. The encapsulation process is performed by the source of the IP packet.

Routing

The network layer provides services to direct the packets to a destination host on another network. To travel to other networks, the packet must be processed by a router. The role of the router is to select the best path and direct packets toward the destination host in a process known as routing. A packet may cross many routers before reaching the destination host. Each router a packet crosses to reach the destination host is called a hop.

In IPv4

The source device uses its own subnet mask along with its own IPv4 address and the destination IPv4 address to make this determination.

Directly-connected networks

These network route entries are active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each router interface is connected to a different network segment. In the figure, the directly-connected networks in the R1 IPv4 routing table would be 192.168.10.0/24 and 209.165.200.224/30.

Remote networks

These network route entries are connected to other routers. Routers learn about remote networks either by being explicitly configured by an administrator or by exchanging route information using a dynamic routing protocol. In the figure, the remote network in the R1 IPv4 routing table would be 10.1.1.0/24.

Remote host

This is a destination host on a remote network. The source and destination hosts do not share the same network address.

Local host

This is a destination host that is on the same local network as the sending host. The source and destination hosts share the same network address.

Best Effort

IP is inherently unreliable because packet delivery is not guaranteed.

IPv4 address depletion

IPv4 has a limited number of unique public addresses available. Although there are approximately 4 billion IPv4 addresses, the increasing number of new IP-enabled devices, always-on connections, and the potential growth of less-developed regions have increased the need for more addresses.

Increased address space

IPv6 addresses are based on 128-bit hierarchical addressing as opposed to IPv4 with 32 bits.

In IPv6

The local router advertises the local network address (prefix) to all devices on the network.

De-encapsulation

When the packet arrives at the network layer of the destination host, the host checks the IP header of the packet. If the destination IP address within the header matches its own IP address, the IP header is removed from the packet. After the packet is de-encapsulated by the network layer, the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer. The de-encapsulation process is performed by the destination host of the IP packet.

Increased network complexity

While NAT has extended the lifespan of IPv4 it was only meant as a transition mechanism to IPv6. NAT in its various implementation creates additional complexity in the network, creating latency and making troubleshooting more difficult.

Eliminates the need for NAT

With such a large number of public IPv6 addresses, NAT between a private IPv4 address and a public IPv4 is not needed. This avoids some of the NAT-induced problems experienced by applications that require end-to-end connectivity.

Connectionless

here is no connection with the destination established before sending data packets.