Chapter 20 - DHCP and IP Networking on Hosts Flashcards
A PC connects to a LAN and uses DHCP to lease an IP address for the first time. Of the usual four DHCP messages that flow between the PC and the DHCP server, which ones do the client send? (Choose two answers.)
a. Acknowledgment
b. Discover
c. Offer
d. Request
B and D.
The client sends a Discover message, with the server returning an Offer message. The client then sends a Request, with the server sending back the IP address in
the Acknowledgment message.
An enterprise puts the DHCP and DNS servers on VLAN 10/subnet 10 in Atlanta, using IP address 10.1.10.1 for the DHCP server and 10.1.10.2 for the DNS server. A remote router sits in Boston, with devices on the Boston LAN using the DHCP and DNS servers in Atlanta. Which of the following needs to be configured in the routers in this enterprise to support DHCP and DNS?
a. The ip helper-address 10.1.10.1 command in the Atlanta router
b. The ip helper-address 10.1.10.2 command in the Boston router
c. The ip name-server 10.1.10.2 command in the Atlanta router
d. The ip dhcp-server 10.1.10.1 command in the Boston router
e. None of the other answers are correct.
E.
Of the commands in the answers, three list syntactically correct commands but one does not: ip dhcp-server 10.1.10.1. The answer with this command is incorrect. The ip helper-address 10.1.10.1 command is indeed needed as an interface subcommand, but only on remote routers like the Boston router. This command is not needed for the DNS server, so while the ip helper-address 10.1.10.2 command would be accepted, it does not help DNS or DHCP work. Finally, the ip name-server 10.1.10.2 command on the Atlanta router works, but it simply lets users of the Atlanta router CLI make use of the DNS server; it has no effect on user DNS traffic. The Boston router would need an ip helper-address 10.1.10.1 command, so that the Boston router could forward DHCP requests to the DHCP server in Atlanta.
Fred decides to migrate from an old DHCP server platform to use a Cisco router at the headquarters building. This DHCP server, created with configuration in IOS on a Cisco router, supports 200 remote subnets. Which of the following settings are made outside of a per-subnet pool of addresses?
a. Client IP address
b. Addresses in that subnet excluded from being leased by the server
c. Default router
d. DNS server
e. Length of address lease
B.
The configuration puts the per-client settings in a DHCP pool: client IP address, mask, default router, and DNS server IP addresses. The pool also lists the length of time for which the client can keep the lease for the address. Only the exclude list (ip dhcp exclude-address) sits outside the DHCP pool.
PC1, a new PC, just booted for the first time when connected to an enterprise network. PC1 used DHCP to lease IP address 10.1.1.1, learning mask 255.255.255.0, DNS server 10.9.9.9, and default gateway 10.1.1.2, with a lease time of 7 days. Assume that the PC has sent nothing else over the network because the DHCP process completed. The user then opens a web browser and types www.ciscopress.com. Which of the following actions happens next?
a. PC1 sends an ARP request to find the web server’s MAC address.
b. PC1 sends an ARP request to find the default gateway’s (10.1.1.2) MAC address.
c. PC1 sends a DNS request about www.ciscopress.com to the DNS server (10.9.9.9).
d. PC1 sends an IP packet to the www.ciscopress.com web server IP address.
B.
If a host does not yet have an IPv4 address assigned to it, the first action should be to try to lease an address with DHCP. The question begins as if that process had completed, but also that nothing else had happened on the network. That statement is meant to say that no ARPs had flows, no DNS messages had flowed, and certainly no user messages.
When the user types www.ciscopress.com into his browser window, PC1 needs to resolve the name into its IPv4 address. However, the DNS, 10.9.9.9, is on a different subnet than PC1. PC1 does not learn an ARP entry for the default router during the DHCP process. (Remember, all those DHCP messages sent by the DHCP client are not sent to the router’s MAC address.) So, although PC1’s next primary action is to resolve the name www.ciscopress.com to learn its IP address, the literal next message sent by PC1 is an ARP request. As a result, PC1 learns R1’s MAC address, and can then forward the DNS request to R1.
Of the incorrect answers, after the ARP request to find the default router’s MAC address, PC1 will send a DNS request, and then send an IP packet to the server’s IP address. And if the server just happens to be in the same subnet as PC1, PC1 would also ARP to find the web server’s IP address.
A packet is sent to a destination address. That single packet is routed through the network to the final router, which then sends the packet to all hosts connected to that one subnet. Which of the following IP address types was used as the destination IP address?
a. A unicast address
b. A network broadcast address
c. A subnet broadcast address
d. A multicast address
C.
By definition, a subnet broadcast (a packet sent to a subnet broadcast address), also called a directed broadcast, is routed like any other packet until it arrives at a router connected to that subnet. For that last forwarding step, the router encapsulates the IP packet in an Ethernet broadcast frame (destination MAC address FFFF.FFFF.FFFF), so that all hosts in the destination subnet receive a copy.
A unicast packet would not be forwarded as a data link broadcast at the last step.
A network broadcast would be replicated by different routers as needed so that it was delivered to all subnets in the classful network.
A multicast packet would be delivered to a subset of subnets, depending on which subnets had hosts that had formerly asked to receive packets sent to that particular multicast address.
A packet is sent to a destination address. That single packet is replicated by a few routers based on the routers’ knowledge of hosts that had earlier registered to receive packets sent to that destination address. The routers do not forward copies of the packet onto subnets where no hosts have registered, and do forward copies of the packet onto subnets where at least one host has registered to receive those packets. Which of the following IP address types was used as the destination IP address?
a. A unicast address
b. A network broadcast address
c. A subnet broadcast address
d. A multicast address
D.
By definition, a multicast IP packet (a packet sent to a Class D IP multicast address) is copied as necessary by routers to make extra copies and forwarded to multiple routers as needed, but not all. The logic revolves around prior knowledge of a host registration process by which hosts declare their interest in receiving packets sent to a particular multicast IP address. Routers exchange this information, so that when a new multicast packet arrives, the routers know where to send copies of that particular multicast packet and where to not bother to send the packet (because no hosts in that part of the network registered to receive a copy).
A unicast packet would not be forwarded as a data link broadcast at the last step.
A network broadcast would be replicated by different routers as needed so that it was delivered to all subnets in the classful network.
A subnet broadcast would be routed as a single packet to the final router in the path. That router would then forward the IP packet as a LAN broadcast so that all hosts in that subnet would receive a copy.
