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Lenuta Alboaie [email protected]


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Computer Networks– http://www.info.uaic.ro/~computernetworks

Network Level

Lenuta Alboaie [email protected]




• Network Level

• IPv4 Problem

• Context

• Characteristics

• Subnets

• Private Networks


• Address Resolution

• IPv6 - overview

• Details -> Future Course


Computer Networks– http://www.info.uaic.ro/~computernetworks


Initial Situation

Before the Internet, only nodes from the same network could communicate with each other


Figure: Individual Network



• Problem

– How to carry packages in a heterogeneous environment?


• At lower levels: how to make the interconnection of a large number of independent


• At higher levels: how to provide support for a wide variety of applications?

Scaling: how could we handle a large number of nodes and


Computer Networks– http://www.info.uaic.ro/~computernetworks


IP – Internet Protocol



Network Level

IP protocol is used for autonomous systems (AS - Autonomous Systems) in order to interconnect

Figure: Internet - collection of interconnected networks


Computer Networks– http://www.info.uaic.ro/~computernetworks

Network Level

• Role: offers connectionless services to transport datagrams from source to

destination; source and destination can be in different networks

• Each datagram is independent from the others

• This level does not guarantee the right transmission (loss, multiplier,…)

• +…Future Course



IP Protocol

IPv4 Datagram



Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IPv4 Datagram

• Common values for Version field are:

• 4 – IP Protocol (RFC 791)

(6 for IPv6 protocol (RFC 1883))



Figure: IPv4 Datagram

Specify the

datagram header length

Specify the size of the entire datagram


IP Protocol

IPv4 Datagram

Type of service the field allows the host to communicate to the subnet (e.g.

routers) what type of service is desired


Figure: Type of Service Field

Figure: IPv4 Datagram


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IPv4 Datagram

Identification field allows the host to

identify if the received segment is part of one datagram



Figure: IPv4 Datagram

• Flags:

DF (Don’t Fragment) – rooters can’t fragment the indicated datagram

MF (More Fragments) – signals that the packet is a fragment, followed by others; last fragment has this bit 0

Fragment offset field - represents the fragment placed in a datagram


IP Protocol

IP Datagram

• Datagram's Fragmentation:

• Each fragment has the same structure as the IP datagram

• Reassembly of datagrams is performed by the receiver

• If a fragment of a datagram is lost, the datagram is

destroyed (an ICMP - Internet Control Message Protocol message is sent to the sender)

• Fragmentation mechanism has been used for some attacks - firewall piercing (a "special“ fragment is considered as

part of a connection already established, so that it can pass through a firewall)


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IP Datagram

• Datagram Filtering:

• It is accomplished by a firewall: it allows access from the outside in the internal network,

according to some policy, certain types of packets ((used by certain protocols / services)

• Forestall a series of attacks regarding security

• The firewall can be software or hardware

• The firewall can function as a proxy or a gateway



IP Protocol

Proxy- role and architecture:

– Indirect access to other networks (Internet) to hosts on the local network (via proxy)

– The proxy allows an Internet connection sharing – The proxy can be software or hardware

– May play roles such us: firewall or cache server

– Used to improve the performance (e.g., caching, flow

control), filtering messages, ensuring anonymity


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IPv4 Datagram

TTL (Time to Live) field specifies the lifetime of the package;

the number is

decremented by every router through which the packet passes



Figure: IPv4 Datagram


IP Protocol

IPv4 Datagram

Protocol field specifies the protocol (from the superior level)

intended to process the datagram


 1 ICMP (Internet Control Message Protocol)

 2 IGMP (Internet Group Message Protocol)

 6 TCP (Transmission Control Protocol)

 17 UDP (User Datagram Protocol)

Figure: IPv4 Datagram


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IPv4 Datagram

Header checksum field used for

detection; if an error occurs the datagram is




Figure: IPv4 Datagram


IP Protocol

IPv4 Datagram

Options Field


Figure: IPv4 Datagram

Options Details

Security Mention if the datagram is a “secret” one

Strict source routing Show full path to go

Loose source routing Indicates a list of routers that should not be skipped

Record route Each rooter adds its own IP


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IPv4 Datagram

Source address and Destination

address fields indicate the

source address and the destination




Figure: IPv4 Datagram


IP Protocol

IPv4 Addresses

• Each IP address includes a network identifier(NetID) and a host identifier (HostID)

• Each network interface has a single IPv4 address

• An IPv4 address has a length of 32 bits

• Initially (RFC 791) there was a division into

network classes: A,B,C,D,E


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IPv4 Addresses


[Computer Networks, 2003 Andrew S. Tanenbaum]


IP Protocol

IPv4 Addresses

• Class A: 128 possible networks, 224 hosts/network

• Class B: 214 possible networks, 216 hosts/network

• Class C: over 2 million networks, 255 hosts/network

• Network Identifier(NetID) is assigned by a central authority (NIC – Network Information Center)

• Host Identifier(HostID) is assigned locally by a network administrator

• Example: – Class A (in decimal notation convention)

0101 0101 0111 1010 0001 0111 1001 0001

• For IPv6, hexadecimal representation is recommended


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

IPv4 Addresses

• An interface network has assigned a unique IP address

• A host can have multiple NICs, therefore it has multiple IP addresses

• The hosts of the same network have the same network identifier (the same NetID)

Broadcast addresses have HostID’s bites equaled to 1

• The IP address in which all HostID’s bites are 0 is called a network address – refers to the hole network

• Example: (network address for a host such us and

• – loopback address (localhost)



IP Protocol

IPv4 Addresses

• From the address space, some addresses are reserved: (RFC 1918):

• –

• – (private addresses)

• – (loopback addresses)

• - (private addresses)

• - (private addresses)

• Private addresses : addresses that are not accessible to the outside (the "real“Internet), but only in the organization's intranet


Computer Networks– http://www.info.uaic.ro/~computernetworks

Private Networks

• Aspects:

• The exponential growth of the hosts number

• Not all hosts offer resources available on the Internet Solution: NAT (Network Address Translation) – RFC

3022, 4008

• The private addresses can be reused (RFC 1918)

• It is based on replacing the private IP address with a public IP address (IP masquerading)



Private Networks

• Functionality:

Figure: NAT mechanism

Routers normally ignore datagrams containing private addresses => private IP addresses can be used in the

organization's intranet

Access to the outside (the "real“ Internet ) is achieved via a gate (mediating gateway) that rewrites the

source IP addresses / destination


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

Subnets using network masks

– It appeared as a solution to the problem of IP address space`s exhaustion

– Simplify Routing

– Subnets cannot be detected externally


Figure: A campus network


IP Protocol

Subnets using network masks

• Division into subnetworks will be made via the network mask(netmask): NetID bits are 1, HostID bits are 0

• Subnet identifier (SubnetID) is generally used to group computers based on physical topology

Example. One way to create a subnet in a B network


Computer Networks– http://www.info.uaic.ro/~computernetworks

IP Protocol

Subnets using network masks


Let’s consider the IP address: 10100000 00000000 00000110 00000111

Network mask:

11111111 11111111 11111100 00000000

Network address:

10100000 00000000 00000100 00000000

Network address = network mask AND IP address

Default subnet masks: - Class A - Class B - Class C



IP Protocol

Convention mark:

x.x.x.x/m means that we apply an m bits mask to the IP specified x.x.x.x address

• Example:

• – it applies a 12-bits mask to address, we select possible values for the last 20 bits (=32-12)

• – it applies a 20-bit mask to address


Computer Networks– http://www.info.uaic.ro/~computernetworks

Network Level

• Protocols

• ICMP (RFC 792)

• ARP (RFC 826)

• RARP (RFC 903)

• BOOTP (RFC 951,1048,1084)


• From IPv4 to IPv6



ICMP Protocol

ICMP – Internet Control Message Protocol

• Used to exchange control messages

• Use IP

• ICMP messages are processed by the IP software, not by the user processes

• Messages types

Message Type Description

8 Echo Request Ask if a host is active 0 Echo Replay “Yes, I’m active”

3 Destination Unreachable The package can’t be delivered (e.g. DF is set) 5 Redirect The message is not correctly routed

11 Time Exceeded Time elapsed (TTL=0) <- (e.g. loop, congestions, low values for time)


Computer Networks– http://www.info.uaic.ro/~computernetworks

Protocolul ICMP

• Used by:

• ping command (Packet Internet Gropher)

• traceroute command

• A package with TTL=1 (1 hop) is sent

• The first router ignores the packet and sends back an ICMP message “time-to-live exceeded”

• A package with TTL=2 is sent (2 hops)

• The second router ignores the packet and sends back an ICMP message “time-to-live exceeded”

• Repeat until it has received a response from the

destination or has reached the maximum number of hops



Address resolution

IP addresses <-> hardware addresses (physical)

The process of finding the hardware address of a host, knowing its IP address is called address resolution(address resolution) – ARP protocol (RFC 826)

ARP –broadcast protocol (each host receives a request for a

physical address, and the answer is given by the one in question)

The process of finding the IP address based on the hardware address is called reverse address resolution –RARP Protocol (RFC 903)

Used to boot workstations without disks

BOOTP (RFC 951,1048,1084)

DHCP (Dynamic Host Configuration Protocol) RFC 2131,2132


Computer Networks– http://www.info.uaic.ro/~computernetworks


• Context:

– Issues in IPv4 addresses world:

• The exponential growth of the hosts` number

• Very large routing tables

• Complex configurations, more and more users (and increasing)

• Failure to ensure QoS

– Pressure from mobile operators




• Objectives for a new protocol:

– Support for billions of hosts – Reducing routing tables

– Simplifying Protocol

– Support for mobile hosts

– Compatibility with the old IP

– Support for future developments of the Internet

– RFC 2460, 2553


Computer Networks– http://www.info.uaic.ro/~computernetworks



• 6 June 2012



• Aspects:

– IPv6 addresses are 16 bytes in length - 2128 addresses

– Note: 16 hexadecimal numbers, 2 digits each, separated by “:”

• Example: 2001:0db8:0000:0000:0000:0000:1428:57ab

• If one or more groups of 4 digits is 0000, the zeros may be omitted and replaced (once) with "::“

• Example: 2001:0db8::1428:57ab

– To maintain compatibility, public IP addresses are considered a subset of IPv6 address space

– IPv4 addresses in IPv6 can be written as: -> :: or 0:0:0:0:0:0:A00:1


Computer Networks– http://www.info.uaic.ro/~computernetworks


• ICMPv6

– ICMP provides functions (reporting data transmission, errors, etc.) plus:

• Neighbor Discovery(Neighbor Discovery Protocol – NDP) - Replaces the ARP

• Multicast listener discovery(Multicast Listener Discovery) – replaces IGMP (Internet Group Management Protocol)

– Details in RFC 4443




• ... More -> Optional Course


Computer Networks– http://www.info.uaic.ro/~computernetworks


• Network Level

• IPv4 Problem

• Context

• Characteristics

• Subnets

• Private Networks


• Address Resolution

• IPv6 - overview

• Details -> Future Course






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