Which of the following best describes the functions of the TTL hop limit for IP package?

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Terms in this set (60)

Addressing end devices

n 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.

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.

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.

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.

Connectionless

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

Best Effort (unreliable)

Packet delivery is not guaranteed.

Media Independent

Operation is independent of the medium carrying the data.

Fragmentation

In some cases, an intermediate device, usually a router, must split up a packet when forwarding it from one medium to a medium with a smaller MTU. This process is called _________ the packet or ____________.

IP Header

Identifies the packet characteristics.

Payload

Contains the Layer 4 segment information and the actual data.

Version

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

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. 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.

Time-to-Live (TTL)

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.

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).

Source IP Address -

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

Destination IP Address

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

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).

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.

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.

Fragment Offset

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

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.

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.

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.

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.

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.

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.

Integrated security

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

-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

The IPv6 simplified header offers several advantages over IPv4:

Version

This field contains a 4-bit binary value identifying the IP packet version. For IPv6 packets, this field is always set to 0110.(IPv6 packet header)

Traffic Class

This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field. It also contains a 6-bit Differentiated Services Code Point (DSCP) value used to classify packets and a 2-bit Explicit Congestion Notification (ECN) used for traffic congestion control.(IPv6 packet header)

Flow Label

This 20-bit field provides a special service for real-time applications. It can be used to inform routers and switches to maintain the same path for the packet flow so that packets are not reordered.(IPv6 packet header)

Payload Length

This 16-bit field is equivalent to the Total Length field in the IPv4 header. It defines the entire packet (fragment) size, including header and optional extensions.(IPv6 packet header)

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. (IPv6 packet header)

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.(IPv6 packet header)

Source Address

This 128-bit field identifies the IPv6 address of the sending host.(IPv6 packet header)

Destination Address

This 128-bit field identifies the IPv6 address of the receiving host.(IPv6 packet header)

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.

Local host

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

Remote host

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

default gateway

The router connected to the local network segment is referred to as the_______.
It is the device that routes traffic from the local network to devices on remote networks. In a home or small business environment, the_________is often used to connect the local network to the Internet.

Direct connection

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

Local network route

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

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.

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.

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.

Network Destination

Lists the reachable networks.

Netmask

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

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.

Interface

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

Metric

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

If

Lists the interface numbers from the Interface List section of the netstat -r command. The interface numbers correspond to the network capable interface on the host, including Ethernet, Wi-Fi, and Bluetooth adapters.

Metric

Lists the cost of each route to a destination. Lower numbers indicate preferred routes.

Network Destination

Lists the reachable networks.

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.

Branch

Teleworkers, small business, and medium-size branch sites. Includes Cisco 800, 1900, 2900, and 3900 Integrated Series Routers (ISR) G2 (2nd generation).

WAN

Large businesses, organizations, and enterprises. Includes the Cisco Catalyst 6500 Series Switches and the Cisco Aggregation Service Router (ASR) 1000.

Service Provider

Large service providers. Includes Cisco ASR 1000, Cisco ASR 9000, Cisco XR 12000, Cisco CRS-3 Carrier Routing System, and 7600 Series routers.

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