ccna textbook

ccna textbook.No other information is modified at each hop. However, this is not the case for Layer 2 frames, which are different in each network segment. Therefore, to send a packet to a PC in another network segmentPC in another segment, the source PC must use a Layer 2 frame whose destination MAC address is the default gateway °Switch A forwards this frame directly from port 3 to Router A. Router A receives this frame and checks the destination MAC address and finds that it is the same as the MAC address of the local interface. Router A strips the Ethernet frame header and passes the IP packet up the TCP/IP stack. Because the destination IP address is different from the address of its interface 1, Router A looks at its routing table and finds that the destination IP address is in the subnet 10.0.2.0/24 to which its interface 2 is connected. Therefore, Router A knows that to forward this frame to 10.0.2.3, it must learn the MAC address of the DNS server. Since Router A does not have this MAC address in its ARP table, it must use ARP to obtain it. Note that a hub connects Router A, Router B, and the DNS server together, and that a hub is a physical layer device that receives frames from Router A and copies the signal to all other interfaces. As a result, both Router B and the DNS server will receive that ARP request. Unlike a switch, a hub does not perform the learning process because it does not look at Layer 2 information, but only at physical layer signals.The DNS server adds Router A to its ARP table, Chapter 9 Sending and Receiving TCP/IP Packetsand sends an ARP reply to Router A that contains the DNS server's MAC address.Router A can now forward DNS queries to the DNS server using the information shown in PDU 2 in Table 9.2. Note that compared to PDU 1, the only difference in PDU 2 is the Ethernet frame header information, the original IP packet and the encapsulated UDP data segment remain the same.After the DNS server receives this Ethernet frame, its NIC finds that its MAC address is the same as the target MAC address, so it strips the Ethernet header and passes the IP packet up the protocol stack. the Internet layer compares the target IP address with the DNS server's IP address, finds that they are the same, and finds that the protocol is UDP, so it passes the data segment up to the The transport layer finds that the target port number is UDP. The transport layer finds that the target port number is 53 and knows that it should pass the DNS query to the DNS application for processing.The DNS server looks up the name and sends back a ^DNS answer containing the IP address 10.0.3.2 of PC.B. The PDU3 in Table 9.1 illustrates the Ethernet header, IP header, and UDP header for this answer. The default gateway configured for the DNS server is assumed to be Router A. Two things should be noted here: first, the destination MAC address is the outgoing port of the Ethernet segment to which Router A - the DNS server belongs; second, the source and destination UDP ports are the opposite of the DNS query. The source port is the port used by the source device, in this case 53, because the connection is directed to this port; the destination port is 50000, on which PC-A listens for the returned UDPDNS answer.After receiving this frame, Router A performs a MAC address comparison, strips the Ethernet header, performs a route lookup, finds the destination connected to interface 1, finds the destination MAC address in the ARP table, and then re-encapsulates the DNS answer into a new Ethernet frame with a source MAC address of 0000.1111.BBBB, and a destination MAC address of 0000.1111.AAAA, as in PDU 40 of Table 9.1 Switch A intelligently forwards this frame out of port 1. PC.A receives this frame and hands the IP packet to layer 3, which hands the data segment to layer 4, which finds the destination port to be 50000. PC.A compares this port with the connection table and finds that this is the DNS answer it is waiting for, and thus learns the IP address of PC-B.When checking connectivity, first ping the target host. If there is no response, then ping the loopback interface of the current device: ping127.0.0.1o If successful, TCP/IP is correctly installed on the current host. next, ensure that the current host is assigned theIP address provided by the DHCP server (using the Windows command ipconfig), and then ping the IP address of the current host; if