3.7 Data Access
4) Which of the following statements are true regarding network layer and data link layer addresses? (Choose three.) - Data link layer addresses are logical and network layer addresses are physical. - Network layer addresses are expressed as 12 hexadecimal digits and data link layer addresses are decimal. - Network layer addresses are logical and data link addresses are expressed as 12 hexadecimal digits. - Data link layer addresses are physical and network layer addresses are logical. - Network layer addresses are either 32 or 128 bits in length. - Data link layer addresses are 32 bits in length.
- Network layer addresses are logical and data link addresses are expressed as 12 hexadecimal digits. - Data link layer addresses are physical and network layer addresses are logical. - Network layer addresses are either 32 or 128 bits in length.
The IP packet contains two IP addresses:
- Source IP address- The IP address of the sending device, which is the original source of the packet. - Destination IP address- The IP address of the receiving device, which is the final destination of the packet.
5) What is the order of the two addresses in the data link frame? - source MAC, destination MAC - destination MAC, source IP - destination IP, source IP - destination MAC, source MAC - source IP, destination IP
- destination MAC, source MAC
What is used to determine the network portion of an IPv4 address? - subnet mask - MAC address - right-most part of the IP address - left-most part of the MAC address
- subnet mask
An IP address contains two parts:
1) Network portion (IPv4) or Prefix (IPv6)- The left-most part of the address that indicates the network in which the IP address is a member. All devices on the same network will have the same network portion of the address. 2) Host portion (IPv4) or Interface ID (IPv6)- The remaining part of the address that identifies a specific device on the network. This portion is unique for each device or interface on the network. Note: The subnet mask (IPv4) or prefix-length (IPv6) is used to identify the network portion of an IP address from the host portion.
IP address
An IP address is the network layer, or Layer 3, logical address used to deliver the IP packet from the original source to the final destination, as shown in the figure.
2) True or false? The right-most part of an IP address is used to identify the network that a device belongs to.
False
True or False? Data Link addresses are physical so they never change in the data link frame from source to destination.
False
Network layer source and destination addresses
Responsible for delivering the IP packet from the original source to the final destination, which may be on the same network or a remote network.
Data link layer source and destination addresses
Responsible for delivering the data link frame from one network interface card (NIC) to another NIC on the same network.
Devices on the Same Network
Source IPv4 address- The IPv4 address of the sending device, the client computer PC1: 192.168.1.110. Destination IPv4 address- The IPv4 address of the receiving device, FTP server: 192.168.1.9. Notice in the figure that the network portion of the source IPv4 address and the network portion of the destination IPv4 address are the same and therefore; the source and destination are on the same network.
The IP packet is encapsulated in a data link frame that contains the following data link information:
Source data link address - The physical address of the NIC that is sending the data link frame. Destination data link address - The physical address of the NIC that is receiving the data link frame. This address is either the next hop router or the address of the final destination device.
Layer 2 data link protocol
The data link Layer 2 physical address has a different role. The purpose of the data link address is to deliver the data link frame from one network interface to another network interface on the same network. As the IP packet travels from host-to-router, router-to-router, and finally router-to-host, at each point along the way the IP packet is encapsulated in a new data link frame. Each data link frame contains the source data link address of the NIC card sending the frame, and the destination data link address of the NIC card receiving the frame. The Layer 2, data link protocol is only used to deliver the packet from NIC-to-NIC on the same network. The router removes the Layer 2 information as it is received on one NIC and adds new data link information before forwarding out the exit NIC on its way towards the final destination.
Network portion (IPv4) or Prefix (IPv6)
The left-most part of the address that indicates the network in which the IP address is a member. All devices on the same network will have the same network portion of the address.
Host portion (IPv4) or Interface ID (IPv6)
The remaining part of the address that identifies a specific device on the network. This portion is unique for each device or interface on the network.
True or false? Frames exchanged between devices in different IP networks must be forwarded to a default gateway.
True
Role of the Data Link Layer Addresses: Different IP Networks
When the sender and receiver of the IP packet are on different networks, the Ethernet data link frame cannot be sent directly to the destination host because the host is not directly reachable in the network of the sender. The Ethernet frame must be sent to another device known as the router or default gateway. Source MAC address - The Ethernet MAC address of the sending device, PC1. The MAC address of the Ethernet interface of PC1 is AA-AA-AA-AA-AA-AA. Destination MAC address - When the receiving device, the destination IP address, is on a different network from the sending device, the sending device uses the Ethernet MAC address of the default gateway or router. In this example, the destination MAC address is the MAC address of the R1 Ethernet interface, 11-11-11-11-11-11. This is the interface that is attached to the same network as PC1, as shown in the figure. The Ethernet frame with the encapsulated IP packet can now be transmitted to R1. R1 forwards the packet to the destination, Web Server. This may mean that R1 forwards the packet to another router or directly to Web Server if the destination is on a network connected to R1. It is important that the IP address of the default gateway be configured on each host on the local network. All packets to a destination on remote networks are sent to the default gateway. Ethernet MAC addresses and the default gateway are discussed in more detail in other modules.
Ethernet Media Access Control (MAC)
When the sender and receiver of the IP packet are on the same network, the data link frame is sent directly to the receiving device. On an Ethernet network, the data link addresses are known as Ethernet Media Access Control (MAC) addresses, as highlighted in the figure. MAC addresses are physically embedded on the Ethernet NIC. Source MAC address - This is the data link address, or the Ethernet MAC address, of the device that sends the data link frame with the encapsulated IP packet. The MAC address of the Ethernet NIC of PC1 is AA-AA-AA-AA-AA-AA, written in hexadecimal notation. Destination MAC address - When the receiving device is on the same network as the sending device, this is the data link address of the receiving device. In this example, the destination MAC address is the MAC address of the FTP server: CC-CC-CC-CC-CC-CC, written in hexadecimal notation.
Role of the Network Layer Addresses
When the sender of the packet is on a different network from the receiver, the source and destination IP addresses will represent hosts on different networks. This will be indicated by the network portion of the IP address of the destination host. Source IPv4 address - The IPv4 address of the sending device, the client computer PC1: 192.168.1.110. Destination IPv4 address - The IPv4 address of the receiving device, the server, Web Server: 172.16.1.99. Notice in the figure that the network portion of the source IPv4 address and destination IPv4 address are on different networks.
