chapter 4, slide: 1 cs 372 – introduction to computer networks* friday july 23, 2010...
TRANSCRIPT
Chapter 4, slide: 1
CS 372 – introduction to computer networks*Friday July 23, 2010
Announcements:
Midterms are graded. Lab 4 is posted.
Acknowledgement: slides drawn heavily from Kurose & Ross
* Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.
Internet addresses
A key aspect of a virtual network is a single, uniform address format
Can't use hardware addresses because different technologies have different address formats
Can't use addresses that are local to a network because multiple networks might use the same addresses internally
Address format must be independent of any particular hardware address format
Chapter 4, slide: 2
IP address notation
IP address is just a 32-bit number It’s the same internally, regardless of its external
representation In decimal form, the range is [0 … 4294967296] Some are reserved In hexadecimal form, the range is [00000000 …
FFFFFFFF]• divides naturally into 4 2-hexdigit groups• e.g.: 80 FD 28 1C• each group represents one byte (octet)• Binary form is 10000000 11111101 00101000 00011100
For convenience, use dotted decimal notation e.g.: 128.253.40.28 (= 80.FD.28.1C) range is [0.0.0.0 … 255.255.255.255]
Chapter 4, slide: 3
Chapter 4, slide: 4
IP Addressing: introduction IP address: 32-bit
identifier for host, router interface
interface: connection between host/router and physical link multiple interfaces per
router one interface per host one IP address per
interface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
Chapter 4, slide: 5
Subnets IP address:
subnet part (higher bits) host part (lower bits)
What’s a subnet ? device interfaces with
same subnet part of IP address
can physically reach each other without intervening router
network consisting of 3 subnets
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
subnet11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/23
Chapter 4, slide: 6
Subnets 223.1.1.0/24223.1.2.0/24
223.1.3.0/24
Recipe To determine the
subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. Subnet mask: /24
Chapter 4, slide: 7
SubnetsHow many? 223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Chapter 4, slide: 8
IP addresses: how to get one?
Q: How does host get IP address?
hard-coded by system admin in a file
DHCP: Dynamic Host Configuration Protocol: dynamically get IP address from as server when
joining the network IP address can be reused by other hosts if released Can renew IP addresses if stayed connected
Chapter 4, slide: 9
DHCP client-server scenario
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
BE
DHCP server
arriving DHCP client needsaddress in thisnetwork
Chapter 4, slide: 10
IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
Chapter 4, slide: 11
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes
IP address format
Each IP address is divided into a prefix and a suffix
Prefix identifies the network and the type of network to which a host computer is attached
Suffix identifies a host computer within that network
Usually includes indicator for number of bits used for prefix
Address format enables efficient routingChapter 4, slide: 12
IP address hierarchy
Every network in a TCP/IP internet is assigned a unique network number
Each host on a specific network is assigned a host number or host address that is unique within that network
Host's IP address is the combination of the network number (prefix) and host address (suffix)
Chapter 4, slide: 13
IP address assignment
Network numbers (prefixes) are unique Host addresses (suffixes) may be
duplicated on different networks The combination of network number prefix
and host address suffix is unique in the entire internet
Chapter 4, slide: 14
IP address assignment
Assignment of network numbers must be coordinated globally
Assignment of host addresses can be managed locally
Chapter 4, slide: 15
IP address design
IP-v4 designers chose 32-bit addresses Allocate some bits for prefix, some for
suffix Large prefix, small suffix - many networks,
few hosts per network Small prefix, large suffix - few networks,
many hosts per network Because of the wide variety of
technologies, need to allow for both large and small networks
Chapter 4, slide: 16
IP addressing (two types):Classful addressing: A, B, C
A: /8 B: /16 C: /24
CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network
portion of address
11001000 00010111 000100 00 00000000
networkpart
hostpart
200.23.16.0/22
(only 28 networks, but 224 hosts per network)
(216 networks, and 216 hosts per network)(224 networks, but only 28 hosts per
network)
Chapter 4, slide: 17
IP address classes: Classful addressing
Multiple address formats that allow both large and small prefixes
Each format is called an address class The class of an address is identified by
first four bits The number of bits allocated for the
prefix is determined by the class
Chapter 4, slide: 18
IP address classes: Classful addressing
Class A, B and C are primary classes Used for ordinary host addressing
Class D is used for multicast, a limited form of broadcast Internet hosts join a multicast group Packets are delivered to all members of group Routers manage delivery of single packet from source to
all members of multicast group Used for multicast backbone
Class E is reserved
Chapter 4, slide: 21
Determining IP class
Dotted decimal makes separating network address from host address easier
Look at first dotted decimal number, and use this table:
Chapter 4, slide: 22
How many networks? Classful scheme does not yield equal number of
networks in each class class A:
First bit must be 0 7 remaining bits identify Class A net 27 (= 128) possible class A nets
• Minus a few that are reserved class B:
First 2 bits must be 10 14 remaining bits identify Class B net 214 (= 16384) possible class B nets
• Minus a few that are reserved class C:
First 3 bits must be 110 21 remaining bits identify Class C net 221 (= 2097152) possible class C nets
• Minus a few that are reservedChapter 4, slide: 23
What if the form doesn't fit?
Large organizations may not be able to get as many addresses in the Internet as they need
Example - UPS needs addresses for millions of computers
Example – School needs 6000 hosts Too big for class C, too many wasted addresses
for class B
Chapter 4, slide: 25
Possible solutions
Classless addressing allow division between prefix and suffix at any bit
boundary Sharing an IP address
Use one IP address for multiple hosts
Chapter 4, slide: 26
Classless Addressing Example
128.193.47.25 dotted decimal80 C1 2F 19 hexadecimal1000 0000 1100 0001 0010 1111 0001 1001 binary
This can be “re-aligned” to use variable-size prefixes and suffixes.
Example: Suppose we want a 22-bit prefix and a 10-bit suffix
Chapter 4, slide: 27
Example: 1000 0000 1100 0001 0010 1111 0001 1001 Logical AND with a "mask" using 22 bits for
prefix (netmask), 10 bits for suffix:1111 1111 1111 1111 1111 1100 0000 0000
… gives a prefix:1000 0000 1100 0001 0010 1100 0000 0000
The complement mask (hostmask)0000 0000 0000 0000 0000 0011 1111 1111
… gives a suffix:0000 0000 0000 0000 0000 0011 0001 1001
Chapter 4, slide: 28
Example:1000 0000 1100 0001 0010 1100 0000 00000000 0000 0000 0000 0000 0011 0001 1001i.e., the “network number” is 80C12C00h, and the
“host number” within the network is 319h With 10 bits for suffix, 1024 host addresses
are available within the subnet (but 2 of these are reserved)
Address still looks almost the same in dotted decimal 128.193.47.25 / 22 Additional information is provided so addressing
can be handled by routers
Chapter 4, slide: 29
CIDR
CIDR (Classless Inter-Domain Routing) address includes specification for number of bits to use for the netmask Example: host address 128.193.47.25/22 What is the netmask?
• 255.255.252.0 What is the network address?
• 128.193.44.0 What is the hostmask?
• 0.0.3.255 What is the host number?
• 0.0.3.25 = 319h = 793 (decimal)
Chapter 4, slide: 30
CIDR
What is the netmask for /20 ? 255.255.240.0
What is the netmask for /24 ? 255.255.255.0
What is the netmask for /27 ? 255.255.255.224
How many hosts can be supported in /28 ? 24 - 2 = 14
Chapter 4, slide: 31
Chapter 4, slide: 32
ExampleSubnet 1
Subnet 2
Subnet 3
Three subnets All interfaces in all these
subnets are required to have prefix: 223.1.13/24
Subnet 1 is required to support 125 interfaces
Subnet 2 & 3 are each required to support 60 interfaces
Question: Provide 3 network addresses in the form: a.b.c.d/x
IP address assignment Select an address class for each network
depending on expected number of hosts Assign network numbers from appropriate classes Assign host suffixes to form internet addresses
for all hosts The lowest host number (0) is not used because
that complete address is the network address e.g., 192.168.5.0
The highest host number (depends on network type) is not used because that complete address is the “broadcast address” for the network e.g., 192.168.5.255
Chapter 4, slide: 33
OSU IP addresses
Oregon State has a single Class B network: 128.193.0.0
All hosts at OSU have 128.193 prefix E.G.:
ns1.oregonstate.edu 128.193.0.10 ns2.oregonstate.edu 128.193.4.20
Suffix bytes are used to determine local network and host through subnetting
Individual host addresses assigned by system administrators may be static or dynamic assignment
Chapter 4, slide: 34
IP address allocation
Addresses in the Internet are not used efficiently Less than 10% of possible addresses are
actually assigned Concerns about address space being
exhausted OSU is like most organizations, using
5,000-6,000 out of possible 216 (= 65,536) available addresses
Chapter 4, slide: 35
Routing table information
In the routing table Destination stored as network address Next hop stored as IP address of router
Address mask defines how many bits of address are in prefix Prefix defines how much of address used to
identify network e.g., class B mask is 255.255.0.0
• In binary: 11111111111111110000000000000000
Chapter 4, slide: 36
Consider 192.4.10.26:
Routing table for
doesn’t care what does with the packets Chapter 4, slide: 38