Cisco Chapter 6 Networking

The IPv6 simplified header offers several advantages over IPv4:

-Better routing efficiency for performance and forwarding-rate scalability -No requirement for processing checksums -Simplified and more efficient extension header mechanisms (as opposed to the IPv4 Options field) -A Flow Label field for per-flow processing with no need to open the transport inner packet to identify the various traffic flows

Itself

A host can ping itself by sending a packet to a special IPv4 address of 127.0.0.1 which is referred to as the loopback interface. This loopback address is automatically assigned to a host when TCP/IP is running. The ability for a host to send a packet to itself using network functionality is useful for testing purposes. Any IP within the network 127.0.0.0/8 refers to the local host.

Internet routing table expansion

A routing table is used by routers to make best path determinations. As the number of servers (nodes) connected to the Internet increases, so too does the number of network routes. These IPv4 routes consume a great deal of memory and processor resources on Internet routers.

Destination IP Address

Contains a 32-bit binary value that represents the destination IP address of the packet.

Source IP Address

Contains a 32-bit binary value that represents the source IP address of the packet.

Version

Contains a 4-bit binary value identifying the IP packet version. For IPv4 packets, this field is always set to 0100.

Internet Header Length (IHL)

Contains a 4-bit binary value identifying the number of 32-bit words in the header. The IHL value varies due to the Options and Padding fields. The minimum value for this field is 5 (i.e., 5×32 = 160 bits = 20 bytes) and the maximum value is 15 (i.e., 15×32 = 480 bits = 60 bytes).

Time-to-Live

Contains an 8-bit binary value that is used to limit the lifetime of a packet. It is specified in seconds but is commonly referred to as hop count. The packet sender sets the initial time-to-live (TTL) value and is decreased by one each time the packet is processed by a router, or hop. If the TTL field decrements to zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message to the source IP address. The traceroute command uses this field to identify the routers used between the source and destination.

Payload

Contains the Layer 4 segment information and the actual data.

Differentiated Services

Formerly called the Type of Service (ToS) field, the DS field is an 8-bit field used to determine the priority of each packet. The first 6 bits identify the Differentiated Services Code Point (DSCP) value that is used by a quality of service (QoS) mechanism. The last 2 bits identify the explicit congestion notification (ECN) value that can be used to prevent dropped packets during times of network congestion.

IP address depletion

IPv4 has a limited number of unique public IP 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. This dramatically increases the number of available IP addresses.

Integrated security

IPv6 natively supports authentication and privacy capabilities. With IPv4, additional features had to be implemented to do this.

C

Identifies a directly connected network. Directly connected networks are automatically created when an interface is configured with an IP address and activated.

Route source

Identifies how the route was learned.

L

Identifies that this is a link local route. Link local routes are automatically created when an interface is configured with an IP address and activated.

Next-hop

Identifies the IP address of the next router to forward the packet.

Destination network

Identifies the address of the remote network.

Outgoing interface

Identifies the exit interface to use to forward a packet toward the final destination.

IP Header

Identifies the packet characteristics.

Administrative distance

Identifies the trustworthiness of the route source.

Route timestamp

Identifies when the route was last heard from.

Addressing end devices

In the same way that a phone has a unique telephone number, end devices must be configured with a unique IP address for identification on the network. An end device with a configured IP address is referred to as a host.

Netmask

Lists a subnet mask that informs the host how to determine the network and the host portions of the IP address.

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

Interface

Lists the address of the physical interface used to send the packet to the gateway that is used to reach the network destination.

Gateway

Lists the address used by the local computer to get to a remote network destination. If a destination is directly reachable, it will show as "on-link" in this column.

Gateway

Lists the address used by the local host to forward packets to a remote network destination. On-link indicates that the host is currently connected to it.

Metric

Lists the cost of each route and is used to determine the best route to a destination.

Network Destination

Lists the reachable networks

Network Destination

Lists the reachable networks.

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 IP address. However, because the public IP address is shared, the IP address of an internal network host is hidden. This can be problematic for technologies that require end-to-end connectivity.

Connectionless

No connection with the destination is established before sending data packets.

Media Independent

Operation is independent of the medium carrying the data.

Best Effort (unreliable)

Packet delivery is not guaranteed. (Packet Characteristic)

Total Length

Sometimes referred to as the Packet Length, this 16-bit field defines the entire packet (fragment) size, including header and data, in bytes. The minimum length packet is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes.

Header Checksum

The 16-bit field is used for error checking of the IP header. The checksum of the header is recalculated and compared to the value in the checksum field. If the values do not match, the packet is discarded

Improved packet handling

The IPv6 header has been simplified with fewer fields. This improves packet handling by intermediate routers and also provides support for extensions and options for increased scalability/longevity.

Local default route

The default route represents the route that packets must take to reach all remote network addresses. The default route is created when a default gateway address is present on the host. The default gateway address is the IP address of the network interface of the router that is connected to the local network. The default gateway address can be configured on the host manually or learned dynamically.

Routing

The network layer provides services to direct 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 paths for and direct packets toward the destination host in a process known as routing. A packet may cross many intermediary devices before reaching the destination host. Each route the packet takes to reach the destination host is called a hop.

Encapsulation

The network layer receives a protocol data unit (PDU) from the transport layer. In a process called encapsulation, the network layer adds IP header information, such as the IP address of the source (sending) and destination (receiving) hosts. After header information is added to the PDU, the PDU is called a packet.

Local network route

The network which the host is connected to is automatically populated in the host routing table.

Remote routes

These routes come from remote networks connected to other routers. Routes to these networks can either be manually configured on the local router by the network administrator or dynamically configured by enabling the local router to exchange routing information with other routers using dynamic routing protocols.

Directly-connected routes

These routes come from the active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each of the router's interfaces is connected to a different network segment. Routers maintain information about the network segments that they are connected to within the routing table.

Destination Address

This 128-bit field identifies the IPv6 address of the receiving host.

Source Address

This 128-bit field identifies the IPv6 address of the sending host.

Fragment Offset

This 13-bit field identifies the order in which to place the packet fragment in the reconstruction of the original unfragmented packet.

Identification

This 16-bit field uniquely identifies the fragment of an original IP packet.

Flags

This 3-bit field identifies how the packet is fragmented. It is used with the Fragment Offset and Identification fields to help reconstruct the fragment into the original packet

Protocol

This 8-bit binary value indicates the data payload type that the packet is carrying, which enables the network layer to pass the data to the appropriate upper-layer protocol. Common values include ICMP (1), TCP (6), and UDP (17).

Next Header

This 8-bit field is equivalent to the IPv4 Protocol field. It indicates the data payload type that the packet is carrying, enabling the network layer to pass the data to the appropriate upper-layer protocol. This field is also used if there are optional extension headers added to the IPv6 packet.

Remote host

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

Local host

This is a host on the same network as the sending host. The hosts share the same network address.

Direct connection

This is a route to the loopback interface (127.0.0.1).

::1/128

This is equivalent to the IPv4 loopback address and provides services to the local host.

::/0

This is the IPv6 equivalent of the local default route.

2001::/32

This is the global unicast network prefix.

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. This process of removing headers from lower layers is known as de-encapsulation. 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.

Eliminates the need for NAT

With such a large number of public IPv6 addresses, Network Address Translation (NAT) is not needed. Customer sites, from the largest enterprises to single households, can get a public IPv6 network address. This avoids some of the NAT-induced application problems experienced by applications requiring end-to-end connectivity.

Hop Limit

this 8-bit field replaces the IPv4 TTL field. This value is decremented by one by each router that forwards the packet. When the counter reaches 0 the packet is discarded and an ICMPv6 message is forwarded to the sending host, indicating that the packet did not reach its destination.