CCNA LS CH6

Destination IP Address [IPv4]

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

Source IP Address [IPv4]

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

Version [IPv4]

Contains a 4-bit binary value set to 0100 that identifies this as an IP version 4 packet.

Time-to-Live (TTL)

Contains an 8-bit binary value that is used to limit the lifetime of a packet. The packet sender sets the initial TTL value, and it is decreased by one each time the packet is processed by a router. 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.

Next Hop [RNRT]

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

Administrative Distance [RNRT]

Identifies the administrative distance (i.e., trustworthiness) of the router source. Lower values indicate increased trustworthiness of the route source.

Destination Network

Identifies the destination network and how it was learned.

Destination Network [RNRT]

Identifies the destination network.

Outgoing Interface

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

Outgoing Interface [RNRT]

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

Metric [RNRT]

Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Route Time stamp [RNRT]

Identifies when the router was last heard from.

IPv4 Router Routing Table

In addition to providing routing information for directly-connected networks and remote networks, the routing table also has information on how the route was learned, the trustworthiness and rating of the route, when the route was last updated, and which interface to use to reach the requested destination.

Network Layer Protocols

Internet Protocol version 4 (IPv4) Internet Protocol version 6 (IPv6)

default gateway

Is the network device that can route traffic to other networks. It is the router that can route traffic out of the local network. A host's routing table will typically include a default gateway.

default route

Is the route or pathway your computer will take when it tries to contact a remote network. The default route is derived from the default gateway configuration and is placed in the host computer's routing table.

WAN

Large businesses, organizations, and enterprises. Includes the Cisco Catalyst Series Switches and the Cisco Aggregation Services Routers (ASR).

Service Provider

Large service providers. Includes Cisco ASR, Cisco CRS-3 Carrier Routing System, and 7600 Series routers.

Default route

Like a host, routers also use a default route as a last resort if there is no other route to the desired network in the routing table.

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.

IP connectionless

Meaning that no dedicated end-to-end connection is created before data is sent. IP requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded.

Network Address Translation (NAT)

NAT provides a way for multiple devices to share a single public IPv4 address.

Lack of end-to-end connectivity

Network Address Translation (NAT) is a technology commonly implemented within IPv4 networks. 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.

Connectionless

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

router components

- Central processing units (CPU) - Operating systems (OS) - Memory consisting of random-access memory (RAM), read-only memory (ROM), nonvolatile random-access memory (NVRAM), and flash.

Global unicast IPv6 packet

2001:6f8x.xxxx.xxxx.

Dropped

A Packet that is not forwarded.

Host Forwarding Decision 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. Pinging the loopback interface tests the TCP/IP protocol stack on the host.

Router Memory

A router has access to volatile or non-volatile memory storage.

Remote Network Routing Table Entries

A router typically has multiple interfaces configured. The routing table stores information about both directly-connected networks and remote networks.

Internet routing table expansion

A routing table is used by routers to make best path determinations. As the number of servers connected to the Internet increases, so too does the number of network routes.

#netstat -r

Another command that can be used to display the host routing table. Displays three sections related to the current TCP/IP network connections:

#route print

Command can be used to display the host routing table. Displays three sections related to the current TCP/IP network connections:

IP Protocol

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

Internet Engineering Task Force (IETF)

Developed of IP version 6 (IPv6). IPv6 overcomes the limitations of IPv4 and is a powerful enhancement with features that better suit current and foreseeable network demands.

Addressing end devices

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

Differentiated Services (DS)

Formerly called the Type of Service (ToS) field, the DS field is an 8-bit field used to determine the priority of each packet.

Heatsink

Helps dissipate the heat generated by the CPU.

Best Effort

IP is inherently unreliable because packet delivery is not guaranteed.

IP address depletion

IPv4 has a limited number of unique public IPv4 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.

Directly-connected network.

Identified by the letter "C" Directly-connected networks are automatically created when an interface is configured with an IP address and activated. Directly connected networks with a route source of C and L have no next-hop address. This is because a router can forward packets directly to hosts on these networks using the designated interface.

Local interface.

Identified by the letter "L" This is the IPv4 address of the interface on the router. Local interface entries did not appear in routing tables prior to IOS Release 15.

Route Source [RT]

Identifies how the network was learned by the router.

Route Source [RNRT]

Identifies how the network was learned by the router. Common route sources include S (static route), D (Enhanced Interior Gateway Routing Protocol or EIGRP), and O (Open Shortest Path First or OSPF). Other route sources are beyond the scope of this chapter.

Media Independent

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

32-bit IPv4 address space

Provides approximately 4,294,967,296 unique addresses.

Rotuer CPU

Requires and executes OS instructions, such as system initialization, routing functions, and switching functions.

Volatile memory

Requires continual power to maintain its information. When the router is powered down or restarted, the content is erased and lost.

Non-volatile memory

Retains its information even when a device is rebooted.

Branch

Teleworkers, small businesses, and medium-size branch sites. Includes Cisco Integrated Services Routers (ISR) G2 (2nd generation).

fragmented packet

The IPv4 packet uses Identification, Flags, and Fragment Offset fields to keep track of the fragments. A router may have to fragment a packet when forwarding it from one medium to another with a smaller MTU.

Improved packet handling

The IPv6 header has been simplified with fewer fields. Consists of 40 octets (largely due to the length of the source and destination IPv6 addresses) and 8 header fields (3 IPv4 basic header fields and 5 additional header fields).

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.

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.

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 the best path 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 router a packet crosses to reach the destination host is called a hop.

Cisco Internetwork Operating System (IOS)

The system software used for most Cisco devices regardless of the size and type of the device. It is used for routers, LAN switches, small wireless access points, large routers with dozens of interfaces, and many other devices.

Remote routes

These routes come from remote networks connected to other routers. Routes to these networks can 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 a dynamic routing protocol.

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.

Payload Length [IPv6]

This 16-bit field indicates the length of the data portion or payload of the IPv6 packet.

Flow Label [IPv6]

This 20-bit field suggests that all packets with the same flow label receive the same type of handling by routers.

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).

Traffic Class [IPv6]

This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field.

Next Header [IPv6]

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.

Hop Limit [IPv6]

This 8-bit field replaces the IPv4 TTL field. This value is decremented by a value of 1 by each router that forwards the packet. When the counter reaches 0, the packet is discarded, and an ICMPv6 Time Exceeded message is forwarded to the sending host, indicating that the packet did not reach its destination because the hop limit was exceeded.

Version [IPv6]

This field contains a 4-bit binary value set to 0110 that identifies this as an IP version 6 packet.

IP header

This header is used to deliver the packet to the destination host. The IP header remains in place from the time the packet leaves the source host until it arrives at the destination host.

Host Forwarding Decision

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

Host Forwarding Decision Local host

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

RAM

This is volatile memory used in Cisco routers to store applications, processes, and data needed to be executed by the CPU. Cisco routers use a fast type of RAM called synchronous dynamic random access memory (SDRAM)

NVRAM

This memory is used as the permanent storage for the startup configuration file (startup-config).

ROM

This non-volatile memory is used to store crucial operational instructions and a limited IOS. Specifically, ROM is firmware embedded on an integrated circuit inside the router which can only be altered by Cisco.

Internet Control Message Protocol (ICMP)

Time Exceeded message to the source IP address.

Internet Header Length (IHL)

Total Length, and Header Checksum fields are used to identify and validate the packet.

Router Packet Forwarding Decision

When a host sends a packet to another host, it will use its routing table to determine where to send the packet. If the destination host is on a remote network, the packet is forwarded to the default gateway. The router looks at its routing table to determine where to forward packets.

Routing Table Forwarding

When a packet arrives at the router interface, the router examines the packet header to determine the destination network. If the destination network matches a route in the routing table, the router forwards the packet using the information specified in the routing table. If there are two or more possible routes to the same destination, the metric is used to decide which route appears in the routing table.

Directly Connected Routing Table Entries

When a router interface is configured with an IPv4 address, a subnet mask, and is activated, the following two routing table entries are automatically created connected network & local interface. It is also important to understand that packets cannot be forwarded by the router without a route for the destination network in the routing table.

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.

extension headers (EH)

Which provide optional network layer information. Extension headers are optional and are placed between the IPv6 header and the payload. EHs are used for fragmentation, security, to support mobility and more.

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 application problems experienced by applications requiring end-to-end connectivity.

connectionless

a connection does not have to be established to send data

default gateway

a device that allows the devices on one network to communicate with devices on other networks

metric

a value that is used by a routing protocol to identify the best route to insert into a routing table

Time-to-Live

a value that limits the the lifetime of a packet

hostname

a way to identify a networking device

IPv4 packet header

consists of fields containing important 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.

transport layer (OSI Layer 4)

manages the data transport between the processes running on each host, network layer protocols specify the packet structure and processing used to carry the data from one host to another host. Operating without regard to the data carried in each packet allows the network layer to carry packets for multiple types of communications between multiple hosts.

flash

nonvolatile memory that stores the IOS and other system files

128-bit IPv6 address space

provides 340,282,366,920,938,463,463,374,607,431,768,211,456, or 340 undecillion addresses, which is roughly equivalent to every grain of sand on Earth.

out-of-band configuration

requires a direct physical connection to the device to be configured

in-band configuration

requires the establishment of a network connection to the device to be configured

next-hop

the destination router indicated by the outgoing interface entry in the routing table

maximum transmission unit

the largest PDU that is supported

routing

the process of sending packets to hosts on a remote network

administrative distance

the trustworthiness of a route

Best Effort Delivery

unreliable oes not have the capability to manage and recover from undelivered or corrupt packets. This is because while IP packets are sent with information about the location of delivery, they contain no information that can be processed to inform the sender whether delivery was successful. Packets may arrive at the destination corrupted, out of sequence, or not at all. IP provides no capability for packet retransmissions if errors occur.

#show ip route

used to display the router's routing table