Title: W4140 Network Laboratory Lecture 3 Sept 18 - Fall 2006 Shlomo Hershkop Columbia University
1W4140 Network LaboratoryLecture 3Sept 18 -
Fall 2006Shlomo HershkopColumbia University
2Announcements
- Lab division
- I will be updating the webpage with lab groups,
if everyone in the room would like to move the
lab around a little, that is ok - Labs reports
- are due generally when the next lab
startscontact me if you need more time. Single
report per group, zipped and uploaded to TA
through courseworks.include relevant information
and name the files using some logical system.
README should include everyones names and cunix
ID - Lab pre-work/post
- courseworks will have the prelabs, which need to
be completed BEFORE your lab startsindividual
work.post labs are to be submitted online
(courseworks) by beginning of next lab (or
earlier please). - Lab 1 pre work was not collected, practice one.
- reading list
- lab 2
- chapter 2 (see resources)
3The Evolution of Internet
Introductory material. An overview lecture that
covers Internet related topics, including a
definition of the Internet, an overview of its
history and growth, and standardization and
naming.
4A Definition
- On October 24, 1995, the FNC unanimously passed a
resolution defining the term Internet.
- RESOLUTION The Federal Networking Council (FNC)
agrees that the following language reflects our
definition of the term "Internet". "Internet"
refers to the global information system that -- - (i) is logically linked together by a globally
unique address space based on the Internet
Protocol (IP) or its subsequent
extensions/follow-ons - (ii) is able to support communications using the
Transmission Control Protocol/Internet Protocol
(TCP/IP) suite or its subsequent
extensions/follow-ons, and/or other IP-compatible
protocols and - (iii) provides, uses or makes accessible, either
publicly or privately, high level services
layered on the communications and related
infrastructure described herein.
5Internet History
1961-1972 Early packet-switching principles
- 1972
- ARPAnet demonstrated publicly
- NCP (Network Control Protocol) first host-host
protocol - first e-mail program
- ARPAnet has 15 nodes
- 1961 Kleinrock - queueing theory shows
effectiveness of packet-switching - 1964 Baran - packet-switching in military nets
- 1967 ARPAnet conceived by Advanced Research
Projects Agency - 1969 first ARPAnet node operational
6Internet History
1972-1980 Internetworking, new and proprietary
nets
- Cerf and Kahns internetworking principles
- minimalism, autonomy - no internal changes
required to interconnect networks - best effort service model
- stateless routers
- decentralized control
- define todays Internet architecture
- 1970 ALOHAnet satellite network in Hawaii
- 1973 Metcalfes PhD thesis proposes Ethernet
- 1974 Cerf and Kahn - architecture for
interconnecting networks - late70s proprietary architectures DECnet, SNA,
XNA - late 70s switching fixed length packets (ATM
precursor) - 1979 ARPAnet has 200 nodes
7Internet History
1990, 2000s commercialization, the Web, new apps
- Early 1990s ARPAnet decommissioned
- 1991 NSF lifts restrictions on commercial use of
NSFnet (decommissioned, 1995) - early 1990s Web
- hypertext Bush 1945, Nelson 1960s
- HTML, HTTP Berners-Lee
- 1994 Mosaic, later Netscape
- late 1990s commercialization of the Web
- Late 1990s 2000s
- more killer apps instant messaging, P2P file
sharing - network security to forefront
- est. 50 million host, 100 million users
- backbone links running at Gbps
8Applications of the Internet
- Traditional core applications Email News Remot
e Login File Transfer - The killer application World-Wide Web (WWW),
P2P - Future applications Videoconferencing and
Telephony Multimedia Services Internet Broadcast
9Growth of the Internet
Source Internet Software Consortium
10Internet Infrastructure
11Internet Infrastructure
- The infrastructure of the Internet consists of a
federation of connected networks that are each
independently managed (autonomous system) - Note Each autononmous system may consist of
multiple IP networks - Hierarchy of network service providers
- Tier-1 nation or worldwide network (US less
than 20) - Tier-2 regional networks (in US less than 100)
- Tier-3 local Internet service provider (in US
several thousand)
12Internet Infrastructure
- Location where a network (ISP, corporate network,
or regional network) gets access to the Internet
is called a Point-of-Presence (POP). - Locations (Tier-1 or Tier-2) networks are
connected for the purpose of exchanging traffic
are called peering points. - Public peering Traffic is swapped in a specific
location, called Internet exchange points (IXPs) - Private peering Two networks establish a direct
link to each other.
13Tier-1 ISP e.g., Sprint
Sprint US backbone network
14Who is Who on the Internet ?
- Internet Society (ISOC) Founded in 1992, an
international nonprofit professional organization
that provides administrative support for the
Internet. Founded in 1992, ISOC is the
organizational home for the standardization
bodies of the Internet. - Internet Engineering Task Force (IETF) Forum
that coordinates the development of new
protocols and standards. Organized into working
groups that are each devoted to a specific topic
or protocol. Working groups document their work
in reports, called Request For Comments (RFCs). - IRTF (Internet Research Task Force) The Internet
Research Task Force is a composed of a number of
focused, long-term and small Research Groups. - Internet Architecture Board (IAB) a technical
advisory group of the Internet Society, provides
oversight of the architecture for the protocols
and the standardization process - The Internet Engineering Steering Group (IESG)
The IESG is responsible for technical management
of IETF activities and the Internet standards
process. Standards. Composed of the Area
Directors of the IETF working groups.
15Internet Standardization Process
- Working groups present their work i of the
Internet are published as RFC (Request for
Comments). - RFCs are the basis for Internet standards.
- Not all RFCs become Internet Standards ! (There
are gt3000 RFCs and less than 70 Internet
standards - A typical (but not only) way of standardization
is - Internet Drafts
- RFC
- Proposed Standard
- Draft Standard (requires 2 working
implementation) - Internet Standard (declared by IAB)
16Assigning Identifiers for the Internet
- Who gives University the domain name netlab.edu
and who assigns it the network prefix
128.143.0.0/16? Who assigns port 80 as the
default port for web servers? - The functions associated with the assignment of
numbers is referred to as Internet Assigned
Number Authority (IANA). - Early days of the Internet IANA functions are
administered by a single person (Jon Postel). - Today
- Internet Corporation for Assigned Names and
Numbers (ICANN) assumes the responsibility for
the assignment of technical protocol parameters,
allocation of the IP address space, management of
the domain name system, and others. - Management of IP address done by Regional
Internet Registries (RIRs) - APNIC (Asia Pacific Network Information Centre)
- RIPE NCC (Réseaux IP Européens Network
Coordination Centre) - ARIN (American Registry for Internet Numbers)
- Domain names are administered by a large number
of private organizations that are accredited by
ICANN.
17Summary
- Layered Internet architecture
- Reduce complexity
- Higher layer views lower layer as service
provider - Application layer, transport layer, network
layer, and link layer
18IP Addressing
- Next
- IP addressing
- Data link protocols and ARP
- Notes about lab
19IP Addressing
- Addressing defines how addresses are allocated
and the structure of addresses - IPv4
- Classful IP addresses (obsolete)
- Classless inter-domain routing (CIDR) (RFC 854,
current standard) - IP Version 6 addresses
20What is an IP Address?
- Why Addresses?
- End-to-end argument (principle)
- Reading
- http//web.mit.edu/Saltzer/www/publications/endtoe
nd/endtoend.pdf - Keep it Simple, Stupid
21What is an IP Address?
- An IP address is a unique global address for a
network interface. - An IP address uniquely identifies a network
location. - http//www.arin.net/whois
- http//www.iana.org/ipaddress/ip-addresses.htm
- Routers forwards a packet based on the
destination address of the packet.
22IPv4 Addresses
23IP v.4 Addresses
24IP v.4 Addressing
- An IP address is often written in dotted decimal
notation - Each byte is identified by a decimal number in
the range 0..255
10001111
10000000
10001001
10010000
1st Byte 128
2nd Byte 143
3rd Byte 137
4th Byte 144
128.143.137.144
25Structure of an IP address
31
0
network prefix
host number
- An IP address encodes both a network number
(network prefix) and an interface number (host
number). - network prefix identifies a network
- the host number identifies a specific host
(actually, interface on the network).
26How long the network prefix is?
- Before 1993 The network prefix is implicitly
defined (class-based addressing) - After 1993 The network prefix is indicated by a
netmask.
27Before 1993 Class-based addressing
- The Internet address space was divided up into
classes - Class A Network prefix is 8 bits long
- Class B Network prefix is 16 bits long
- Class C Network prefix is 24 bits long
- Class D is multicast address
- Class E is reserved
28Classful IP Adresses (Until 1993)
- Each IP address contained a key which identifies
the class - Class A IP address starts with 0
- Class B IP address starts with 10
- Class C IP address starts with 110
- Class D IP address starts with 1110
- Class E IP address starts wit 11110
29The old way Internet Address Classes
30The old way Internet Address Classes
31The old way Internet Address Classes
Class Leading bits Start End CIDR equivalent
Class A 0 0.0.0.0 127.255.255.255 /8
Class B 10 128.0.0.0 191.255.255.255 /16
Class C 110 192.0.0.0 223.255.255.255 /24
Class D (multicast) 1110 224.0.0.0 239.255.255.255 NA
Class E (reserved) 1111 240.0.0.0 255.255.255.255 NA
32Problems with Classful IP Addresses
- Fast growing routing table size
- Each router must have an entry for every network
prefix - 221 2,097,152 class C networks
- In 1993, the size of routing tables started to
outgrow the capacity of routers
33Other problems with classful addresses
- Address depletion for large networks
- Class A and Class B addresses were gone
- How many class A/B network prefixes can there be?
- Limited flexibility for network addresses
- Class A and B addresses are overkill (gt64,000
addresses) - Class C address is insufficient (256 addresses)
34Classless Inter-domain routing (CIDR) 1993
- Full description RFC 1518 1519
- Network prefix is of variable length
- Addresses are allocated hierarchically
- Routers aggregate multiple address prefixes into
one routing entry to minimize routing table size
35CIDR network prefix is variable length
144
16
128
59
10001111
10000000
10001001
10010000
Addr
255
255
0
255
11111111
11111111
1111111
00000000
Mask
- A network mask specifies the number of bits used
to identify a network in an IP address. - How?
36CIDR notation
- CIDR notation of an IP address
- 128.143.137.144/24
- /24 is the prefix length. It states that the
first 24 bits are the network prefix of the
address (and the remaining 8 bits are available
for specific host addresses) - CIDR notation can nicely express blocks of
addresses - An address block
- 128.195.0.0, 128.195.255.255
- can be represented by an address prefix
128.195.0.0/16 - How many addresses are there in a /x address
block? - 2 (32-x)
37CIDR hierarchical address allocation
128.0.0.0/8
ISP
128.59.0.0/16
128.1.0.0/16
128.2.0.0/16
University
128.59.16.150
Foo.com
Bar.com
CS
Library
128.59.16.0/24
128.59.44.0/24
- IP addresses are hierarchically allocated.
- An ISP obtains an address block from a Regional
Internet Registry - An ISP allocates a subdivision of the address
block to an organization - An organization recursively allocates subdivision
of its address block to its networks - A host in a network obtains an address within the
address block assigned to the network
38Hierarchical address allocation
128.59.16.0 255
128.59.16.150
128.59.0.0 128.59.255.255
128.0.0.0 - 128.255.255.255
- ISP obtains an address block 128.0.0.0/8 ?
128.0.0.0, 128.255.255.255 - ISP allocates 128.59.0.0/16 (128.59.0.0,
128.59.255.255) to the university. - University allocates 128.59.16.0/24
(128.59.16.0, 128.59.16.255) to the CS
departments network - A host on the CS departments network gets one IP
address 128.59.16.150
39CIDR allows route aggregation
You can reach 128.0.0.0/8 via ISP1
128.0.0.0/8
ISP3
ISP1
128.1.0.0/16
128.2.0.0/16
128.59.0.0/16
University
Foo.com
Bar.com
CS
Library
- ISP1 announces one address prefix 128.0.0.0./8 to
ISP2 - ISP2 can use one routing entry to reach all
networks connected to ISP1
40CIDR summary
- A network prefix is of variable length
a.b.c.d/x - Addresses are hierarchical allocated
- Routers aggregate multiple address prefixes into
one routing entry to minimize routing table
size. - Security is still an issue
- Secure Routing Path validation
41What problems CIDR does not solve (I)
You can reach 128.0.0.0/8 And 204.1.0.0/16 via
ISP1
ISP3
ISP1
ISP2
128.0.0.0/8
204.0.0.0/8
204.1.0.0/16
Mutil-home.com
204.1.0.0/16
- An multi-homing site still adds one entry into
global routing tables
42What problems CIDR does not solve (II)
You can reach 128.0.0.0/8 And 204.1.0.0/16 via
ISP1
ISP3
ISP1
ISP2
128.0.0.0/8
204.0.0.0/8
128.0.0.0/8 ISP1
204.1.0.0/16
Switched.com
204.1.0.0/16
- A site switches provider without renumbering
still adds one entry into global routing tables
43Global routing tables continue to grow
Source http//bgp.potaroo.net/as4637/
44Special IPv4 Addresses
- Reserved or (by convention) special addresses
- Loopback interfaces
- all addresses 127.0.0.1-127.255.255.255 are
reserved for loopback interfaces - Most systems use 127.0.0.1 as loopback address
- loopback interface is associated with name
localhost - Broadcast address
- Host number is all ones, e.g., 128.143.255.255
- Broadcast goes to all hosts on the network
- Often ignored due to security concerns
- Test / Experimental addresses
- 10.0.0.0 - 10.255.255.255
- 172.16.0.0 - 172.31.255.255
- 192.168.0.0 - 192.168.255.255
- Convention (but not a reserved address)
- Default gateway has host number set to 1, e.g.,
128.195.4.1
45Special IPv4 Addresses (RFC 3330)
Addresses CIDR Equivalent Purpose RFC Class of addresses
0.0.0.0 - 0.255.255.255 0.0.0.0/8 Zero Addresses RFC 1700 A 16,777,216
10.0.0.0 - 10.255.255.255 10.0.0.0/8 Private IP addresses RFC 1918 A 16,777,216
127.0.0.0 - 127.255.255.255 127.0.0.0/8 Localhost Loopback Address RFC 1700 A 16,777,216
169.254.0.0 - 169.254.255.255 169.254.0.0/16 Zeroconf RFC 3330 B 65,536
172.16.0.0 - 172.31.255.255 172.16.0.0/12 Private IP addresses RFC 1918 B 1,048,576
192.0.2.0 - 192.0.2.255 192.0.2.0/24 Documentation and Examples RFC 3330 C 256
192.88.99.0 - 192.88.99.255 192.88.99.0/24 IPv6 to IPv4 relay Anycast RFC 3068 C 256
192.168.0.0 - 192.168.255.255 192.168.0.0/16 Private IP addresses RFC 1918 C 65,536
198.18.0.0 - 198.19.255.255 198.18.0.0/15 Network Device Benchmark RFC 2544 C 131,072
224.0.0.0 - 239.255.255.255 224.0.0.0/4 Multicast RFC 3171 D 268,435,456
240.0.0.0 - 255.255.255.255 240.0.0.0/4 Reserved RFC 1700 E 268,435,456
46IP Addressing (Summary)
- Addressing defines how addresses are allocated
and the structure of addresses - IPv4
- Classful IP addresses (obsolete)
- Classless inter-domain routing (CIDR) (current
standard) - IP Version 6 addresses
47IPv6 - IP Version 6
- IP Version 6
- Designed to be the successor to the currently
used IPv4 - Specification completed in 1994
- Makes improvements to IPv4 (no revolutionary
changes) - One (not the only !) feature of IPv6 is a
significant increase in of the IP address to 128
bits (16 bytes) - IPv6 will solve for the foreseeable future
the problems with IP addressing - 1024 addresses per square inch on the surface of
the Earth.
48IPv6 Header
49Notation of IPv6 addresses
- Convention The 128-bit IPv6 address is written
as eight 16-bit integers (using hexadecimal
digits for each integer) - CEDFBP7632454464FACE2E503025DF12
- Short notation
- Abbreviations of leading zeroes
- CEDFBP7600000000009E00003025DF12 ?
CEDFBP76009E 03025DF12 - 000000000000 can be written as
- CEDFBP7600FACE03025DF12 ?
CEDFBP76FACE03025DF12
50IPv4 address in IPv6
- IPv6 addresses derived from IPv4 addresses have
96 leading zero bits. - Convention allows to use IPv4 notation for the
last 32 bits. - 808F8990 ? 128.143.137.144
51IPv6 vs. IPv4 Address Comparison
- IPv4 has a maximum of
- 232 ? 4 billion addresses
- IPv6 has a maximum of
- 2128 (232)4 ? 4 billion x 4 billion x 4
billion x 4 billion addresses - Is IPv6 widely deployed?
52Data Link Layer
- The main tasks of the data link layer are
- Transfer data from the network layer of one
machine to the network layer of another machine - Convert the raw bit stream of the physical layer
into groups of bits (frames)
53TCP/IP Protocol Stack
- The TCP/IP protocol stack runs on top of multiple
data link layers. - Two data link layer technologies
- Broadcast
- Point-to-Point
54Two types of networks at the data link layer
- Broadcast Networks All stations share a single
communication channel - Point-to-Point Networks Pairs of hosts (or
routers) are directly connected - Typically, local area networks (LANs) are
broadcast and wide area networks (WANs) are
point-to-point
55Local Area Networks
- Local area networks (LANs) connect computers
within a building or a enterprise network - Almost all LANs are broadcast networks
- Typical topologies of LANs are bus or ring or
star - We will work with Ethernet LANs. Ethernet has a
bus or star topology.
56MAC and LLC
- In any broadcast network, the stations must
ensure that only one station transmits at a time
on the shared communication channel - The protocol that determines who can transmit on
a broadcast channel are called Medium Access
Control (MAC) protocol - The MAC protocol are implemented in the MAC
sublayer which is the lower sublayer of the
data link layer - The higher portion of the data link layer is
often called Logical Link Control (LLC)
57IEEE 802 Standards
- IEEE 802 is a family of standards for LANs, which
defines an LLC and several MAC sublayers
Higher layer issues
LLC
CSMA/CS
Token bus
Token ring
Wireless lan
58Ethernet
- Speed 10Mbps -10 Gbps
- Standard 802.3, Ethernet II (DIX)
- Most popular physical layers for Ethernet
- 10Base5 Thick Ethernet 10 Mbps coax cable
- 10Base2 Thin Ethernet 10 Mbps coax cable
- 10Base-T 10 Mbps Twisted Pair
- 100Base-TX 100 Mbps over Category 5 twisted pair
- 100Base-FX 100 Mbps over Fiber Optics
- 1000Base-FX 1Gbps over Fiber Optics
- 10000Base-FX 1Gbps over Fiber Optics (for wide
area links) -
59Bus Topology
- 10Base5 and 10xBase2 Ethernets has a bus topology
60Star Topology
- Starting with 10Base-T, stations are connected to
a hub in a star configuration
61Ethernet Hubs vs. Ethernet Switches
- An Ethernet switch is a packet switch for
Ethernet frames - Buffering of frames prevents collisions.
- Each port is isolated and builds its own
collision domain - An Ethernet Hub does not perform buffering
- Collisions occur if two frames arrive at the same
time.
Hub
Switch
62Ethernet and IEEE 802.3 Any Difference?
- There are two types of Ethernet frames in use,
with subtle differences - Ethernet (Ethernet II, DIX (Digital-Intel-Xerox)
- An industry standards from 1982 that is based on
the first implementation of CSMA/CD by Xerox. - Predominant version of CSMA/CD in the US.
- 802.3
- IEEEs version of CSMA/CD from 1985.
- Interoperates with 802.2 (LLC) as higher layer.
- Difference for our purposes Ethernet and 802.3
use different methods to encapsulate an IP
datagram.
63Ethernet II, DIX Encapsulation (RFC 894)
64IEEE 802.2/802.3 Encapsulation (RFC 1042)
65Point-to-Point (serial) links
- Many data link connections are point-to-point
serial links - Dial-in or DSL access connects hosts to access
routers - Routers are connected by high-speed
point-to-point links - Here, IP hosts and routers are connected by a
serial cable - Data link layer protocols for point-to-point
links are simple - Main role is encapsulation of IP datagrams
- No media access control needed
66Data Link Protocols for Point-to-Point links
- SLIP (Serial Line IP)
- First protocol for sending IP datagrams over
dial-up links (from 1988) - Encapsulation, not much else
- PPP (Point-to-Point Protocol)
- Successor to SLIP (1992), with added
functionality - Used for dial-in and for high-speed routers
- HDLC (High-level Data Link Control)
- Widely used and influential standard (1979)
- Default protocol for serial links on Cisco
routers - Actually, PPP is based on a variant of HDLC
67PPP - IP encapsulation
- The frame format of PPP is similar to HDLC and
the 802.2 LLC frame format -
- PPP assumes a duplex circuit
- Note PPP does not use addresses
- Usual maximum frame size is 1500
68Additional PPP functionality
- In addition to encapsulation, PPP supports
- multiple network layer protocols (protocol
multiplexing) - Link configuration
- Link quality testing
- Error detection
- Option negotiation
- Address notification
- Authentication
- The above functions are supported by helper
protocols - LCP
- PAP, CHAP
- NCP
69PPP Support protocols
- Link management The link control protocol (LCP)
is responsible for establishing, configuring, and
negotiating a data-link connection. LCP also
monitors the link quality and is used to
terminate the link. - Authentication Authentication is optional. PPP
supports two authentication protocols Password
Authentication Protocol (PAP) and Challenge
Handshake Authentication Protocol (CHAP). - Network protocol configuration PPP has network
control protocols (NCPs) for numerous network
layer protocols. The IP control protocol (IPCP)
negotiates IP address assignments and other
parameters when IP is used as network layer.
70Address Resolution Protocol(ARP)
71Overview
72ARP and RARP
- Note
- The Internet is based on IP addresses
- Data link protocols (Ethernet, FDDI, ATM) may
have different (MAC) addresses - The ARP and RARP protocols perform the
translation between IP addresses and MAC layer
addresses - We will discuss ARP for broadcast LANs,
particularly Ethernet LANs
73Processing of IP packets by network device
drivers
74Address Translation with ARP
- ARP Request Argon broadcasts an ARP request to
all stations on the network What is the
hardware address of 128.143.137.1?
75Address Translation with ARP
- ARP Reply Router 137 responds with an ARP Reply
which contains the hardware address
76ARP Packet Format
77Example
- ARP Request from Argon
- Source hardware address 00a02471e444Sourc
e protocol address 128.143.137.144Target
hardware address 000000000000Target
protocol address 128.143.137.1 - ARP Reply from Router137
- Source hardware address 00e0f923a820
Source protocol address 128.143.137.1 Target
hardware address 00a02471e444Target
protocol address 128.143.137.144
78ARP Cache
- Since sending an ARP request/reply for each IP
datagram is inefficient, hosts maintain a cache
(ARP Cache) of current entries. The entries
expire after a time interval. - Contents of the ARP Cache
- (128.143.71.37) at 00104BC5D115 ether on
eth0 - (128.143.71.36) at 00B0D0E117D5 ether on
eth0 - (128.143.71.35) at 00B0D0DE70E6 ether on
eth0 - (128.143.136.90) at 00053C062735 ether on
eth1 - (128.143.71.34) at 00B0D0E117DB ether on
eth0 - (128.143.71.33) at 00B0D0E117DF ether on
eth0
79Proxy ARP
- Proxy ARP Host or router responds to ARP Request
that arrives from one of its connected networks
for a host that is on another of its connected
networks.
80Things to know about ARP
- What happens if an ARP Request is made for a
non-existing host? - Several ARP requests are made with increasing
time intervals between requests. Entually, ARP
gives up (timeout). - On some systems (including Linux) a host
periodically sends ARP Requests for all addresses
listed in the ARP cache. This refreshes the ARP
cache content, but also introduces traffic. - Gratuitous ARP Requests A host sends an ARP
request for its own IP address - Useful for detecting if an IP address has already
been assigned.
81Vulnerabilities of ARP
- Since ARP does not authenticate requests or
replies, ARP Requests and Replies can be forged - ARP is stateless ARP Replies can be sent without
a corresponding ARP Request - According to the ARP protocol specification, a
node receiving an ARP packet (Request or Reply)
must update its local ARP cache with the
information in the source fields, if the
receiving node already has an entry for the IP
address of the source in its ARP cache. (This
applies for ARP Request packets and for ARP Reply
packets)
82Vulnerabilities of ARP
- Typical exploitation of these vulnerabilities
- A forged ARP Request or Reply can be used to
update the ARP cache of a remote system with a
forged entry (ARP Poisoning) - This can be used to redirect IP traffic to other
hosts
83Some notes on Lab 2
84What is a single-segment network?
128.59.2.0/24
128.59.2.100
128.59.1.0/24
128.59.1.100
128.59.2.1
128.59.1.1
128.59.1.200
128.59.3.1
128.59.1.300
128.59.2.200
128.59.3.0/24
128.59.3.200
128.59.3.100
- A single-segment network consists of interfaces
connected by a single physical link, either a
point-to-point link or a broadcast link. - Interfaces on the same single-segment network
have the same network prefix.
85How to identify a single segment IP network
128.59.2.100
128.59.1.100
128.59.2.1
128.59.1.1
128.59.1.200
128.59.3.1
128.59.1.300
128.59.2.200
128.59.3.200
128.59.3.100
- Detach interfaces from routers or hosts
- Each isolated island is a single segment IP
network - Each interface on the same single segment IP
network must have the same network address prefix
86Protocol specification vs implementation
- According to the ARP protocol specification, a
node receiving an ARP packet (Request or Reply)
must update its local ARP cache with the
information in the source fields, if the
receiving node already has an entry for the IP
address of the source in its ARP cache. (This
applies for ARP Request packets and for ARP Reply
packets) - Implementation may differ from the specification
- What you observe in the lab may not be
universally true.