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In the TCP/IP model, the Network Interface layer corresponds to which layer(s) of the OSI model? – Physical and MAC layers

Give three examples of link layer protocols? – IEEE 802.3/Ethernet, Frame Relay, ATM, and SONET

In the TCP/IP layer, the network layer is responsible for enabling the routing of the data across (logical or physical): – It is across logical network path as it uses packet format and IP address format.

What is the min/max length of an IP header? – 20/65535octets

Which field in the IP header is responsible for defining the Quality of Service? – It is TOS (Type of Service)

How to prioritize the forwarding of an IP packet? – The priority is set through the Precedence field of the TOS. The TOS is a part of the DiffServ quality of service model and it is also used by OSPF protocol.

What Per-hop-behaviour (PHB) refers to? – It refers to the queuing and the treatment behaviour of router.

How a router can notify his neighbours that it supports congestion? – By setting to 1 the ECN field (Explicit Congestion Notification)

Provide the definition of MTU and state for which purposes it is used? – It defines the maximum size of a frame in the data link layer. In case a packet exceeds the tolerated size, it is then fragmentized.

In which TCP/IP layer an IP packet are fragmented? And based on which criteria it is reassembled in the correct order? – A packet is fragmented in the network layer. An identifier is added to each fragment.

It always possible that a router fragments a packet or not (we assume that the packet size is higher that the data link capability of a router)? – The fragmentation rule is not absolute. A router can deny fragmentation by setting the flag Don’t Fragment (DF) to 1 (DF=1).

How a receiver detects the end of a fragmented flow (a flow is a set of packets from the same source)? – The receiver detects the end of a fragment flow when it encounter a fragment with MF=0

Does a checksum of an IP packet is recalculated at each router or not? – Yes because the TTL field is decremented at each hop and this field is part of the IP packet header.

How a router or a host determine the network of an IP address? – A router determines the network of a given IPv4 address by using the first octet and the network mask.

What are the two main restrictions that apply for network masks in classful environment? – An IPv4 host address cannot have all its host bits set to 0 (reserved for routers), and the host bits cannot be set to 1 (reserved for broadcast).

How a host discover the data-link identifier of another neighbor? – By using ARP protocol

How an ARP request is sent? – The request is encapsulated into a MAC frame. The source @ of the frame is the MAC @ of the source and the destination @ is the broadcast address (all-hosts address).

Which Cisco command debugs the activity of the ARP protocol? – debug arp

Which Cisco command deletes all the entries of an ARP table? – clear arp-cashe

What is a gratuitous ARP requests? – It is an ARP request issued by a host using its own IPv4 address as the target @. It is used to detect address duplication.

For which purposes the HSRP (Hot Standby Router Protocol) is used for? – It is used for fault tolerance issue. One router is configured to be active and another one is a standby. When the active fails, the standby takes place.

What is a “Source Quench” message? When it is used? – A destination host may send a source quench message if datagrams arrive too fast to be processed.

How hosts discover local routers? – By listening to Router Advertisement and Router Selection messages (ICMP messages, type 9 and 10)

By which means, TCP protocol provide reliability? – TCP uses a system of acknowledgment, checksums, retransmissions and timers to provide reliability.

How the TCP protocol regulates the flow of IP packets? – The TCP protocol uses a mechanism called windowing to control the flow between two pairs and avoid congestion.

What are the essential differences between TCP and UDP? – Correct sequencing, reliability and flow control through the windowing mechanism.

Instead of ARP, Novell NetWare uses a network address that includes a device’s MAC address at the host portion. Why can’t IP do this? – If IP used MAC identifiers as the host part of the IP address, subnetting would not be possible.

Select a subnet mask for so there will be at least 16000 subnets with at least 700 host addresses available on each subnet. – A subnet mask of provides 16,382 subnets of the class A, and 1022 host addresses on each subnet.

Select a subnet mask for so there are at least 500 subnets with at least 100 host addresses available on each subnet. – provides 510 subnets with 126 host addresses on each subnet.

Use a 28-bit mask to derive the available subnets of Then derive the available host addresses of each subnet. –,,, …,