Network Messages’ Encapsulation, Addressing and End-to-end ...

Network Messages’ Encapsulation, Addressing and End-to-end Delivery Process

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Outline

The Header Encapsulation and Addressing of Network Messages

The End-to-end Delivery Process of Network Messages

Summary

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<Simulation Case>network_message_end_to_end_delivery.xtpl

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The Header Encapsulation and Addressing of Network Messages

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The Encapsulation and Decapsulation of Network Messages

5

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver

Network Interface Card (NIC)

LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

1 1 10

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

1 1 10

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

Frame

Bit

Host Host

Layer 5Application Layer

Layer 4Transport Layer

Layer 3Network Layer

Layer 2Data Link Layer

Layer 1Physical Layer


A Network Message’s Encapsulation at the Sending Site

On an Internet device, network application programs are executed to send network messages to the network. Before being sent out, network messages are usually encapsulated three times. The 1st one occurs at layer 4 (transport

layer) and common cases are TCP or UDP encapsulation.

The 2nd one occurs at layer 3 (network layer and common cases are IPv4 or IPv6 encapsulation.

The 3rd one occurs at layer 2 (data link layer). Different network interface cards have different encapsulation methods, such as Ethernet, Wi-Fi, Bluetooth, LTE, etc.

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NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

Frame

1st Encapsulation

2nd Encapsulation

3rd Encapsulation

Layer 5Application

Layer 4Transport

Layer 3Network

Layer 2Data Link

Layer 1Physical


In short, MAC address is the address of a network interface card (NIC). When a network message is going to be sent to a network from a NIC, filling destination MAC address in the message’s MAC header can address a single or multiple NICs on the network.

Generally speaking, if two network hosts are directly connected or connected through layer-1 or layer-2 devices (such as hub or switch), they are considered MAC-addressable to each other. If they are connected through layer-3 devices (such as router), they are not considered MAC-addressable to each other.

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In the graph above, two subnets are separated by the router. In the left subnet, four NIC’s marked with purple squares are MAC-addressable to each other. Similarly, in the right subnet, four NIC’s marked with orange squares are MAC-addressable to each other.

However, a purple-square NIC and a orange-square NIC are not MAC-addressable to each other. That means the addressable range of MAC address is limited within a subnet.

During the encapsulation process at the 2nd layer, a MAC header is prepended to a network message. The MAC header is filled with a

destination MAC address to achieve MAC addressing.


During the encapsulation process at the 3rd layer, an IP header is prepended to a network message. The IP header is filled with a

destination IP address to achieve IP addressing.

Because the addressable range of MAC address is limited within a subnet, a network message requires IP addressing if its destination is located beyond a subnet.

A router, which divides subnets, forwards network messages according to the destination IP address of network messages.

During the encapsulation process of a network message, both IP addressing and MAC addressing are involved.

During the delivery process of a network message, the message, in general, experiences one IP addressing and more than one MAC addressing (if across a subnet).

In the graph above, the host at the left-down corner wants to send a message to the host at the right-down corner. During the delivery process of the message, it experiences one IP addressing the two MAC addressing.

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IP Addressing

MACAddressing

MACAddressing


The End-to-end Delivery Process of Network Messages

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An Example of End-to-end Network Message Delivery Process

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IP Addressing

MACAddressing

IP Address -> 1.0.1.3MAC Address -> 00:01:00:00:00:09



IP Address -> 1.0.2.4MAC Address -> 00:01:00:00:00:0e

Sender Receiver

MACAddressing

HubHost

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

Frame

BitPHY1 1 1

0

Switch

ASIC

LLC / MAC

PHY

Switching Fabric

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Bit

1 1 10

Frame

Router

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

DataTCP/UDP Header

IP Header

DataTCP/UDP Header

IP Header

Device Driver

Routing Logic &Layer 3+ Utility


LLC / MAC

PHY

ARPEther

HeaderData

TCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Frame Frame

Packet Packet

Bit

1 1 10

Bit

1 1 10

Switch

ASIC

LLC / MAC

PHY

Switching Fabric

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Frame FrameFrame

Hub

PHY1 1 10

Bit

1 1 10

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Frame

Packet

Segment

Datagram

Bit


(1) A network application program (layer 5, application layer) running on the sender host has a message to be sent to the receiver host.

The sender host takes the NIC’s IP address of the receiver host as the message’s destination.

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

Data

11

A network application program assigns 1.0.2.4 to be the destination IP address.

IP Addressing

IP Address -> 1.0.2.4Sender Receiver

Layer 5Application


(2) If a network message has to be sent by TCP or UDP protocol (layer 4, transport layer), it has to be encapsulated with a TCP or

UDP header.

After being encapsulated with a TCP header, a network message is also called a TCP segment.

After being encapsulated with a UDP header, a network message is also called a UDP datagram.

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

Segment

Datagram

12

Layer 4Transport



(3) When a network message reaches layer 3 (network layer), it is encapsulated with an IP header.

After being encapsulated with an IP header, a network message is also called a packet.

The destination’s IP address is filled in the IP header.

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

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Layer 3Network

IP Addressing

IP Address -> 1.0.1.1


1) The destination’s IP address 1.0.2.4 is filled in the IP header.

2) Query the network-layer routing table to know that a message has to be forwarded by the router if its destination address is 1.0.2.4, that is located beyond the current subnet. Thus, the message has to be sent to the router’s NIC first whose address is 1.0.1.1.

Sender Receiver



(4) Get the MAC address of a message’s next destination NIC by Address Resolution Protocol (ARP).

The ARP turns the next destination NIC’s IP address into the NIC’s MAC address. The MAC address is filled in the MAC header of the message.

Because the NIC used in this simulation case is an Ethernet NIC, the layer-2 MAC header is called Ether header.

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

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ARP

1) In the IP header, the IP destination address is 1.0.2.4.

2) The IP address of the next destination NIC is 1.0.1.1.

3) Query ARP table to try to get the MAC address of the NIC whose IP address is 1.0.1.1. If no result is obtained, an ARP request is sent to the network to ask the answer.

MACAddressing


Sender Receiver


(4-1) ARP Request and ARP Reply

At the sender site, if no IP-to-MAC address mapping entry is found in the ARP table, the ARP sends a broadcast message to network to ask the answer. This message is called ARP request.

When a NIC receives an ARP request and finds that it is the asking target of the ARP request, it sends back an ARP reply that carries its MAC address.

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< Broadcast ARP Request >Who is 1.0.1.1？

Tell me your MAC address.


Sender Receiver

< Unicast ARP Reply >I am 1.0.1.1.

My MAC address is 00:01:00:00:00:01.


(5) When a network message reaches layer 2 (data link layer), it is encapsulated with a MAC header.

When a message is processed by the ARP, a MAC header is prepended to the message and the destination MAC address is filled in the header. In general, the whole MAC header processing is finished at data link layer (layer 2).

After being encapsulated with a MAC header, a network message is also called a frame.

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

Frame

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Layer 2Data Link

Frame


(6) When a network message reaches layer 1 (physical layer), it is transformed into signals and transmitted through media.

In short, the main procedure at physical layer is signal processing, including signal encoding (transform a frame into signals), signal decoding (transform signals into a frame), and signal amplification, etc.

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Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

Frame

Bit

1 1 10

Layer 1Physical

Frame


HubHost

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Segment

Datagram

Packet

Frame

BitPHY1 1 1

01 1 1

0

(7) When signals reach a layer-1 hub, the hub forwards the signals directly.

On a hub, signals are not transformed into a frame for checking the frame’s MAC header.

A hub amplifies signals’ power and forwards them out. Doing this extends the signal transmission range and increases the rate of successful signal decoding at receiver sites.

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Signal Signal


Hub

PHY

Switch

ASIC

LLC / MAC

PHY

Switching Fabric

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Bit

1 1 10

Frame Frame

Bit

1 1 10

(8) When signals reach a layer-2 switch, the signals are transformed into a frame for checking the frame’s MAC header. The switch retrieves the destination MAC address from a MAC header. The address is used to query the

switch’s forwarding table to determine to which port(s) this frame should be forwarded. If no answer is obtained from the table, the switch forwards the frame to all ports except the input port.

After the output port(s) is determined, the layer-2 frame is again transformed into layer-1 signals and the signals are transmitted.

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Frame Frame

MAC Address ->00:01:00:00:00:01

Sender Receiver

？

？

L2

L1

00:01:00:00:00:01

Signal Signal


(9) When signals reach a layer-3 router, the signals are first transformed into a frame for checking MAC header. Next, the frame’s MAC header is removed so that the frame becomes a packet. Finally, the IP header of the packet is

checked for routing.

After the signals are transformed into a frame, the destination MAC address retrieved from the frame’s MAC header is checked to determine if this frame reaches the MAC-addressing destination or not. If the answer is yes, the MAC header of the frame is removed so that the frame becomes a packet. The packet is passed to layer 3 and the IP header of the packet is checked for routing.

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Switch

ASIC

LLC / MAC

PHY

Switching Fabric

Ether Header

DataTCP/UDP Header

IP Header

Router

Ether Header

DataTCP/UDP Header

IP Header

DataTCP/UDP Header

IP Header

Device Driver



LLC / MAC

PHY

ARPEther

HeaderData

TCP/UDP Header

IP Header

Frame Frame

Packet

Bit

1 1 10

Frame

Packet

Layer 2

Layer 1

Layer 3

Signal


(9-1) MAC Address Checking The network message travels from the sender host to

the left NIC of the router. First, the destination MAC address of the MAC header is

compared with the NIC’s MAC address to confirm that the message reaches the MAC-addressing destination.

Next, the MAC header is removed and the message is passed to layer 3.

Router

Ether Header

DataTCP/UDP Header

IP Header

DataTCP/UDP Header

IP Header

Device Driver



LLC / MAC

PHY

ARPEther

HeaderData

TCP/UDP Header

IP Header

Frame

Packet

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MACAddressing

MAC Address -> 00:01:00:00:00:01

Sender Receiver

Layer 2

Frame00:01:00:00:00:01


(9-2) IP Address Checking and Routing If the prefix of a message is an IP header, the message is

called a packet. When a packet reaches layer 3, the destination IP

address of the IP header is checked to determine if this packet reaches its IP-addressing destination or not. If yes, the message is passed to layer 4. If no, the destination IP address is used to query the routing table to determine to which interface(s) this packet should be forwarded.

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Router

Ether Header

DataTCP/UDP Header

IP Header

DataTCP/UDP Header

IP Header

Device Driver



LLC / MAC

PHY

ARPEther

HeaderData

TCP/UDP Header

IP Header

Frame

Packet Layer 3Packet

IP Addressing -> 1.0.1.1

Sender Receiver

IP地址 -> 1.0.2.4

Has reached the IP-addressing destination Not yet reached,

be forwarded out1.0.2.4


Router

DataTCP/UDP Header

IP Header

Device Driver



LLC / MAC

PHY

ARP

Packet

(10) When a packet/message is being routed at layer 3, its destination IP address is used to query the routing table.

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Layer 3


1) Dst IP address is 1.0.2.4.2) Query the routing table and

know that the destination IP address is located at the same subnet where the router’s right NIC is located. That means the destination NIC and the router’s right NIC are MAC-addressable to each other.

Sender Receiver

A packet has not yet reached the IP-addressing destination. It has to be forwarded.

Packet


Router

DataTCP/UDP Header

IP Header

Device Driver



LLC / MAC

PHY

ARPEther

HeaderData

TCP/UDP Header

IP Header

Packet

(11) Get the MAC address of a message’s next destination NIC by Address Resolution Protocol (ARP).

The ARP turns the next destination NIC’s IP address into the NIC’s MAC address. The MAC address is filled in the MAC header of the message.

Because the NIC used in this simulation case is an Ethernet NIC, the layer-2 MAC header is called Ether header.

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ARP

MACAddressing

Sender Receiver


Packet

1) The IP address of the next destination NIC is 1.0.2.4.

2) Query ARP table to try to get the MAC address of the NIC whose IP address is 1.0.2.4. If no result is obtained, an ARP request is sent to the network to ask the answer.


(11-1) ARP Request and ARP Reply

At the sender site, if no IP-to-MAC address mapping entry is found in the ARP table, the ARP sends a broadcast message to network to ask the answer. This message is called ARP request.

When a NIC receives an ARP request and finds that it is the asking target of the ARP request, it sends back an ARP reply that carries its MAC address.

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< Broadcast ARP Request >Who is 1.0.2.4？

Tell me your MAC address.

Sender Receiver

< Unicast ARP Reply >I am 1.0.2.4.

My MAC Address is 00:01:00:00:00:0e.



Router

Ether Header

DataTCP/UDP Header

IP Header

DataTCP/UDP Header

IP Header

Device Driver



LLC / MAC

PHY

ARPEther

HeaderData

TCP/UDP Header

IP Header

Packet

Frame

1 1 10

Bit

(12) The message is passed down to layer 2 (data link layer) and layer 1 (physical layer). Finally, it is transformed into signals and the signals are

transmitted.

The ways that address resolution protocol, layer 2 protocol, and layer 1 protocol process the message are the same as the ways used on the sender host.

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Layer 2

Layer 1

Packet

Frame

Signal


Router

Ether Header

DataTCP/UDP Header

IP Header

DataTCP/UDP Header

IP Header

Device Driver



LLC / MAC

PHY

ARPEther

HeaderData

TCP/UDP Header

IP Header

Packet

Bit

1 1 10

Switch

ASIC

LLC / MAC

PHY

Switching Fabric

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Frame FrameFrame

1 1 10

Bit

(13) When signals reach a layer-2 switch, the signals are transformed into a frame for checking the frame’s MAC header.

The switch retrieves the destination MAC address from a MAC header. The address is used to query the switch’s forwarding table to determine to which port(s) this frame should be forwarded. If no answer is obtained from the table, the switch forwards the frame to all ports except the input port.

After the output port(s) is determined, the layer-2 frame is again transformed into layer-1 signals and the signals are transmitted out.

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Frame Frame

SenderReceiver

？？

L2

L1

00:01:00:00:00:0e

Dst MAC Address00:01:00:00:00:0e

MAC Address -> 00:01:00:00:00:0e Signal Signal


ASIC

LLC / MAC

PHY

Switching Fabric

Ether Header

DataTCP/UDP Header

IP Header

Frame

Hub

PHY1 1 10

Bit

1 1 10

Bit

(14) When signals reach a layer-1 hub, the hub forwards the signals directly.

On a hub, signals are not transformed into a frame for checking the frame’s MAC header.

A hub amplifies signals’ power and forwards them out. Doing this extends the signal transmission range and increases the success rate of signal decoding at receiver sites.

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Signal Signal


Hub

PHYBit

1 1 10

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Frame

Packet

(15) When signals reach the receiver host, the signals are first transformed into a frame for checking MAC header. Next, the frame’s MAC header is removed so that the frame becomes a packet. Finally, the destination IP

address of the packet is checked.

After the signals are transformed into a frame, the destination MAC address retrieved from the frame’s MAC header is checked to determine if this frame reaches the MAC-addressing destination or not. If the answer is yes, the MAC header of the frame is removed so that the frame becomes a packet. The packet is passed to layer 3 and the destination IP address in the IP header is checked to determine if this packet reaches the IP-addressing destination or not.

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Layer 2

Layer 1

Frame

PacketLayer 3

Signal


Bit

1 1 10

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Frame

Packet

(15-1) MAC Addressing Checking

The network message travels from the right NIC of the router to the receiver host.

First, the destination MAC address of the MAC header is compared with the NIC’s MAC address to confirm that the message reaches the MAC-addressing destination.

Next, the MAC header is removed and the message is passed to layer 3.

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Sender Receiver

MAC Address -> 00:01:00:00:00:0e

Dst MAC Address00:01:00:00:00:0e

00:01:00:00:00:0eFrame

Packet

Layer 2

Layer 1

Layer 3

Signal


(15-2) IP Addressing Checking and Port Number Checking If the prefix of a message is an IP header, the

message is called a packet. When a packet reaches layer 3, the destination IP

address of the IP header is checked to determine if this packet reaches its IP-addressing destination or not. If yes, the IP header of the packet is removed and the rest of the message is passed to layer 4.

The layer-4 TCP or UDP protocol dispatches the message to a layer-5 network application program according to the destination port number carried in the TCP or UDP header.

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Bit

1 1 10

Host

NetworkApplications

TCP / UDP

IPv4 / IPv6

ARP

Device Driver


LLC / MAC

PHY

UserSpace

KernelSpace

Hardware

DataTCP

Header

DataUDP

Header

Data

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Ether Header

DataTCP/UDP Header

IP Header

Frame

Packet

Segment

Datagram

Sender Receiver

Dst IP Address1.0.2.4

（Already reached the IP-addressing destination）


信息包1.0.2.4

Layer 3

Layer 4

Layer 5

Has reached the IP-addressing destination


Summary

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Review of Points

In a network, how far is the addressable range of MAC address? How far is the addressable range of IP address?

During the encapsulation process of a network message, both IP addressing and MAC addressing are involved. Describe 1) how the network layer (layer 3) queries the next destination NIC’s IP address according to the final destination NIC’s IP address? 2) how to translate the next destination NIC’s IP address to its MAC address by ARP?

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Date post:	23-Feb-2022
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