CCNA Exploration Chapter 5 OSI network layer. Study questions. Answers
What are the four main processes at the network layer?
Addressing, encapsulation, routing, decapsulation
What is special about a device that we call a host?
It has an IPv4 address.
What is the name given to the layer 3 protocol data unit?
Encapsulation at the network layer is the addition of a layer 3 header. What addresses are in this header?
Destination IP address and source IP address.
Using the OSI model, which layer is the packet sent to for further encapsulation?
Data link layer, layer 2.
What are the main purposes of a router?
It selects routes and forwards packets towards their destinations.
What is a hop?
The transfer of a packet from one router to the next as part of its journey.
What happens to the contents of a packet after decapsulation?
The contents are passed to the transport layer (layer 4) for further processing.
What network layer protocol is expected to replace IPv4 eventually?
Does IP operate differently on wired and wireless networks?
No, it is independent of the medium. Only layers 1 and 2 are concerned with the medium.
What are the other two basic characteristics of IP?
It is connectionless (does not set up an end-to-end connection before sending data) and best effort (does not check that packets are delivered).
Why was IP designed with these characteristics?
To give it low overhead. IP does not send extra messages to set up a connection. The IP header does not have extra information to check whether or not packets are delivered correctly.
IP is an unreliable protocol. What does this mean?
Unreliable means the same as best effort. It sends data without checking first that the destination exists and can receive the data. It does not check for lost packets or resend lost packets.
Is there a problem because IP is unreliable?
No. If reliability is needed then it can be provided at layer 4 (TCP) or higher. Providing reliability at layer 3 would slow things down.
IP needs one piece of information from the data link layer before it can encapsulate data into packets. What is this information?
The maximum size of packet that can be transported on the medium – the maximum transmission unit (MTU).
The originating (source) host adds the IP header to the transport layer pdu to create the packet. How long do the addresses in this header remain unchanged?
The IP addresses stay unchanged for the whole journey. The IP header is removed by the final destination host.
How many bits make up an IPv4 address?
What is the purpose of the time to live (TTL) field in the packet header?
It stops packets from being forwarded endlessly if they cannot be delivered. Each time the packet reaches a router, the number in the TTL field is decreased. When the number reaches 0, the router will drop the packet.
What is the purpose of the Type of Service field in the IP header?
It allows packets to be given priorities.
What is the purpose of the Protocol field in the IP header?
It shows which layer 4 protocol produced the segment/datagram that was encapsulated. The destination will decapsulate the packet and pass the contents to that layer 4 protocol.
Networks have to be split up because very large networks are too difficult to manage. What are three likely ways of splitting up a network?
Geographically, by purpose and by ownership.
What is a broadcast, and what is a broadcast domain?
A broadcast is a message sent from one host to all other hosts on the network. The broadcast domain is the group of linked devices that receive broadcasts from each other. A network or subnet is often the same thing as a broadcast domain.
What device is used to split up a network into separate broadcast domains (or subnets)?
A router, because it does not pass on broadcasts by default, and therefore limits their spread.
Why is a large broadcast domain inefficient?
The large number of hosts will produce a large number of broadcasts, which will take up bandwidth. Each host has to process each broadcast that it receives, taking up resources on the host.
What is the advantage of splitting up a network on the basis of ownership of data?
Security measures can be put in place using a router or firewall to restrict traffic between the subnets, and thus restrict access to data.
What is the name given to a router that serves as an entry and exit point of a network?
The IP addressing scheme is hierarchical and each IP address has at least two parts. What are these parts?
Network part and host part
Which part of the IP address does a router use when selecting routes and forwarding packets?
The IP address is split into four sets of 8 bits for convenience of reading and writing. What name is given to a set of 8 bits?
Hosts are configured with a default gateway address. What is a default gateway address?
The IP address of a router port that is on the same network as the host. All messages for hosts on other networks should be sent via this port.
Does a host need a default gateway address in order to communicate with other hosts on its network?
No. The default gateway is only needed if messages pass to and from another network.
What must be the same about a host’s own IP address and the address of its default gateway?
The network part of both addresses must be the same.
What is a routing table?
It is a table held in the memory of a router. It lists networks and says which way a packet should be sent to reach each network.
What is the name given to the router that a packet should be forwarded to next?
Next hop router.
When would a router use a default route?
When the router does not have the destination network of a packet listed in its routing table. If the router has a default route configured then packets with unknown destinations will be sent on the default route.
What happens to packets if a router does not have a default route configured?
If the destination network of the packet is listed in the routing table then the packet will be forwarded. Any other packets will be dropped.
If there are several possible routes to an network, will they all be listed in the routing table?
The router selectes the “best” route to put in the routing table. It uses metrics to decide which route is best.
This is part of a routing table.
Gateway of last resort is 192.168.2.2 to network 0.0.0.0
10.0.0.0/24 is subnetted, 2 subnets
R 10.1.1.0 [120/1] via 192.168.2.2, , Fastethernet0/1
R 10.1.2.0 [120/1] via 192.168.2.2, , Fastethernet0/1
C 192.168.1.0/24 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 192.168.2.2
A packet is addressed to 10.1.2.35. Will the router forward this packet? If so, which port will it use, and what is the address of the next-hop router?
Yes, through port FastEthernet0/1 to next hop router 192.168.2.2.
A packet is addressed to 220.127.116.11. Will the router forward this packet? If so, which port will it use, and what is the address of the next-hop router?
The network is not in the routing table, but there is a default route so the router will forward the packet through Fastethernet0/1 to next hop router 192.168.2.2.
A packet is addressed to 192.168.1.20. Will the router forward this packet? If so, which port will it use, and what is the address of the next-hop router?
Yes, through port Fastethernet0/0. There is no next hop router because the network is directly connected and the packet can go straight to the destination.
What is the name for a route that is manually entered into a routing table?
What is the name for a route that is learned by a router from another router using a routing protocol?
What is a routing protocol?
A set of rules used by routers to exchange information about routes.
What are the advantages of using a routing protocol rather than configuring all routes manually?
Once the protocol has been started, the router will learn about routes automatically and will have its information constantly updated without further input from the administrator.
What are the disadvantages of using a routing protocol?
The exchange of messages uses bandwidth. The processing of the information takes up processor time. There may be security implications.