Networking Devices and Initial Configuration Module 7 – 9 Checkpoint Test Online

Hint

When a PC wants to send data on the network, it always knows the IP address of the destination. However, it also needs to discover the MAC address of the destination. ARP is the protocol that is used to discover the MAC address of a host that belongs to the same network.

Hint

Communication inside a local network uses Address Resolution Protocol to obtain a MAC address from a known IPv4 address. A MAC address is needed to construct the frame in which the packet is encapsulated.

Hint

An ARP request is broadcast to all devices on a LAN segment which seek the MAC address for a known IP address. Pings are used to verify connectivity between two devices, and DNS resolves URLs to IP addresses.

Hint

An ARP request is used to determine any unknown MAC address when the destination IP address is known. In an IPv4-based network, this request is sent as a Layer 2 broadcast.

Hint

A frame is encapsulated with source and destination MAC addresses. The source device will not know the MAC address of the remote host. An ARP request will be sent by the source and will be responded to by the router. The router will respond with the MAC address of its interface, the one which is connected to the same network as the source.

Hint

When a network device has to communicate with a device on another network, it broadcasts an ARP request asking for the default gateway MAC address. The default gateway (RT1) unicasts an ARP reply with the Fa0/0 MAC address.

Hint

ARP requests are sent as broadcasts: (1) All nodes will receive them, and they will be processed by software, interrupting the CPU. (2) The switch forwards (floods) Layer 2 broadcasts to all ports. A switch does not change its MAC table based on ARP request or reply messages. The switch populates the MAC table using the source MAC address of all frames. The ARP payload is very small and does not overload the switch.

Hint

The fact that the web server can be accessed by its IP address indicates that the web server is working and there is connectivity between the workstation and the web server. However, the web server domain name is not resolving correctly to its IP address. This could be caused by a misconfiguration of the DNS server IP address on the workstation or the wrong entry of the web server in the DNS server.

Hint

A client will first send the DHCPDISCOVER broadcast message to find DHCPv4 servers on the network. This message will have the limited broadcast address, 255.255.255.255, as the destination address. The client IPv4 address, the default gateway address, and subnet fields will all be 0.0.0.0 because these have not yet been configured on the client. When the DHCPv4 server receives a DHCPDISCOVER message, it reserves an available IPv4 address to lease to the client and sends the unicast DHCPOFFER message to the requesting client. When the client receives the DHCPOFFER from the server, it sends back a DHCPREQUEST broadcast message. On receiving the DHCPREQUEST message, the server replies with a unicast DHCPACK message.

Hint

The ping command tests the connection between two hosts. When ping uses a host domain name to test the connection, the resolver on the PC will first perform the name resolution to query the DNS server for the IP address of the host. If the ping command is unable to resolve the domain name to an IP address, an error will result. Nslookup is a tool for testing and troubleshooting DNS servers.

Hint

When a host configured to use DHCP powers up on a network it sends a DHCPDISCOVER message. FF-FF-FF-FF-FF-FF is the L2 broadcast address. A DHCP server replies with a unicast DHCPOFFER message back to the host.

Hint

A DNS server uses an A record type for an IPv4 end device address. The AAAA record is for an IPv6 end device address. The MX record is used to map the domain name to mail exchange servers. The NS record indicates the authoritative name server.

Hint

Two important functions of DNS are to (1) provide IP addresses for domain names such as www.cisco.com, and (2) forward requests that cannot be resolved to other servers in order to provide domain name to IP address translation. DHCP provides IP addressing information to local devices. A file transfer protocol such as FTP, SFTP, or TFTP provides file sharing services. IMAP or POP can be used to retrieve an email message from a server.

Hint

During the boot process, the PC starts by broadcasting a DHCPDISCOVER message to request an available IP address. A DHCP server replies with a DHCPOFFER message, which offers a lease to the device. This message contains the IP address and other information. The PC may receive multiple DHCPOFFER messages if there is more than one DHCP server. In this case, the PC must choose one of the server DHCP offerings. The PC broadcasts a DHCPREQUEST message that identifies the explicit server and lease offer that the PC is accepting. Then the server will return a DHCPACK message that acknowledges to the PC that the lease is finalized.

Hint

The file server will send back an acknowledgement number of 3001 to host A after receiving the first three segments. The starting sequence number was 1. 1000 bytes were sent in each of the three segments for a total of 3000 bytes. Because the initial sequence number was 1, the last byte sent was byte number 3000. The file server will now expect to receive byte number 3001 and above. Therefore, it communicates to host A through the acknowledgement number of 3001.

Hint

Multiplexing is useful for interleaving multiple communication streams. Window size is used to slow down the rate of data communication. Port numbers are used to pass data streams to their proper applications. Acknowledgments are used to notify a sending device that a stream of data packets has or has not been received.

Hint

The client randomly selects an available source port in the range of the registered ports.

Hint

To establish a session with the client, the TCP server will acknowledge the receipt of the SYN segment from the client. The server sends a segment back to the client with the ACK flag set indicating that the acknowledgment number is significant.The value of the acknowledgment number field is equal to the randomly chosen initial sequence number (ISN) plus 1.

Hint

Together the TCP sequence number and acknowledgment number fields are used by the receiver to inform the sender of the bytes of data that the receiver has accepted.

Hint

A socket is a combination of the source IP address and source port or the destination IP address and the destination port number.

Hint

TCP uses windows to attempt to manage the rate of transmission to the maximum flow that the network and destination device can support while minimizing loss and retransmissions. When overwhelmed with data, the destination can send a request to reduce the of the window. The process of the destination sending acknowledgments as it processes bytes received and the continual adjustment of the source send window is known as sliding windows.

Hint

When a network device has to communicate with a device on another network, it broadcasts an ARP request asking for the default gateway MAC address. The default gateway (RT1) unicasts an ARP reply with the Fa0/0 MAC address.