Subnet & Classless Address Extensions Linda Wu (CMPT 471 2003-3)
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Transcript of Subnet & Classless Address Extensions Linda Wu (CMPT 471 2003-3)
Subnet & Classless Address Extensions
Linda Wu
(CMPT 471 • 2003-3)
Notes-4 CMPT 471 2003-3 2
Content
Motivation Transparent routers Proxy ARP Subnet addressing Classless addressing
Reference: chapter 10
Notes-4 CMPT 471 2003-3 3
Motivation Problem: network growth will exhaust
IPv4 address space eventually Solution: minimize the number of
addresses used Avoid assigning netid whenever possible Share one netid among multiple
networks: transparent router, proxy ARP, subnet addressing
Arbitrary assignment: classless addressing
Notes-4 CMPT 471 2003-3 4
Transparent Routers
A special router T, called transparent router, connects WAN and LAN
Other hosts and routers on the WAN do not know T’s existence
The LAN does not have its own netid; the hosts in the LAN are assigned unused addresses in the WAN
T sends packets from the WAN to the appropriate host in the LAN
T accepts packets from the LAN and routes them across WAN to the destination
Notes-4 CMPT 471 2003-3 5
Transparent Routers (cont.)
Advantages One netid is shared by multiple networks:
fewer network addresses are required Support load balancing
Disadvantages Only work with WAN that has a large
address space Do not provide all the same services as
conventional routers
Notes-4 CMPT 471 2003-3 6
Proxy ARP ARP
Address Resolution Protocol Maps IP addresses to physical addresses
Proxy an application that closes a straight path between
2 networks and prevents the crackers from obtaining internal addresses and details of a private network
A B CR
D E
Main network
Hidden network
Router running proxy ARP
Notes-4 CMPT 471 2003-3 7
Proxy ARP (cont.)
How proxy ARP works? Main network and hidden network: share the
same netid A router, R, connects these 2 networks R knows which hosts lie on which physical
networks, and maintains the illusion that only one network exists
A in the main network sends packets to E in the hidden network
A broadcasts ARP request for E’s physical addr. R responses ARP request by sending back its own
physical addr. A sends the packet destined for E to R R forwards the packet destined for E over the hidden
network
Notes-4 CMPT 471 2003-3 8
Proxy ARP (cont.)
Advantages One netid is shared by multiple networks Proxy ARP can be added to a single
router without disturbing other hosts or routers on the network
Disadvantages The network must use ARP for address
resolution Cannot be generalized to more complex
network topology Rely on managers to maintain tables of
machines and addresses manually
Notes-4 CMPT 471 2003-3 9
Subnet Addressing Subnet addressing, subnetting A network is divided into several
smaller subnets Each subnet has its own subnet
address Subnets appear as a single network
to the rest of the internet The router attached to the subnets
knows the network is physically divided into subnets
Notes-4 CMPT 471 2003-3 10
Subnet Addressing (cont.)
Subnetting Example
To the rest of the internet
141.14.128.1 141.14.128.2
141.14.5.1 141.14.5.2 141.14.5.3 141.14.64.1 141.14.64.5 141.14.64.7
R1
R2netid: 141.14.0.0Class B
Subnet: 141.14.5.0 Subnet: 141.14.64.0
Subnet: 141.14.128.0
Notes-4 CMPT 471 2003-3 11
Subnet Addressing (cont.)
Subnetting address
141 • 14 • 191 • 10
netid hostid
Without subnetting
141 • 14 • 192 • 192
netid subnetid hostid
With subnetting
( 405 ) 864 - 8902
Area code Exchange Connection
Hierarchy in telephone number
Hierarchy addressing:
netid
subnetid
hostid
Notes-4 CMPT 471 2003-3 12
Subnet Addressing (cont.)
Mask a 32-bit binary number that gives the network
address when bitwise ANDed with an IP address e.g.
IP address: 123.24.3.1 (class B)Mask: 11111111 11111111 00000000 00000000IP & mask = 123.24.0.0 (network address)
Bitwise AND
mask
IP address
Network address
Notes-4 CMPT 471 2003-3 13
Subnet Addressing (cont.)
Default masksMasks for class A, B, C addresses 1s: preserve the netid 0s: set the hostid to 0 Number of 1s is predetermined: 8/16/24
Class Binary mask Dotted-decimal mask
A 11111111 00000000 00000000 00000000 255.0.0.0
B 11111111 11111111 00000000 00000000 255.255.0.0
C 11111111 11111111 11111111 00000000 255.255.255.0
Notes-4 CMPT 471 2003-3 14
Subnet Addressing (cont.)
Subnet mask Change some of the leftmost 0s in the
default mask to 1s to make a subnet mask Preserve netid and subnetid, set hostid to 0 Contiguous subnet mask (recommended)
11111111 11111111 11000000 00000000 Noncontiguous subnet mask
11111111 11111111 00110000 001000000
Subnet mask: 255.255.224.0
11111111 11111111 111 00000 00000000
Default mask: 255.255.0.0
11111111 11111111 00000000 00000000
Notes-4 CMPT 471 2003-3 15
Subnet Addressing (cont.)
Bitwise AND
Subnet mask
255.255.224.0
141.14.72.24
IP address
141.14.64.0
Network address
72 010 01000
224 111 00000
010 00000 (64)
Notes-4 CMPT 471 2003-3 16
Subnet Addressing (cont.)
Subnet design exampleA company is granted network address 200.16.64.0 (class C). It needs 6 subnets. Design the subnet. # of 1s in the default mask = 24 (class C) 6 subnets < 23: need 3 more 1s in the subnet
mask Total # of 1s in the subnet mask: 24 + 3 = 27 Total # of 0s in the subnet mask: 8 – 3 = 5 (hostid
bits) Mask is: 11111111 11111111 11111111 11100000,
or, 255.255.255.224 # of hosts per subnet: 25 = 32
Notes-4 CMPT 471 2003-3 17
Subnet Addressing (cont.) Subnet address ranges
1st: 200.16.64.0 ~ 200.16.64.31
2nd: 200.16.64.32 ~ 200.16.64.63
3rd : 200.16.64.64 ~ 200.16.64.95
4th : 200.16.64.96 ~ 200.16.64.127
5th : 200.16.64.128 ~ 200.16.64.159
6th : 200.16.64.160 ~ 200.16.64.191
7th : 200.16.64.192 ~ 200.16.64.223
8th : 200.16.64.224 ~ 200.16.64.255
Notes-4 CMPT 471 2003-3 18
Subnet Addressing (cont.)
Fixed-length subnetting All 1s or all 0s subnet is not recommended All 1s and all 0s host addresses are reserved
Variable-length subnetting No single subnetid partition works for all
organizations An organization may select subnetid partition on
a per-network basis; all hosts and routers attached to the network must follow the partition
Subnet bits # of subnets Hosts per subnet (class B)
0 1 (20) 65534 (216 – 2)
2 2 (22-2) 16382 (214 – 2)
8 254 (28-2) 254 (28-2)
Notes-4 CMPT 471 2003-3 19
Subnet Addressing (cont.)
Variable-length subnetting exampleA class C site has 5 subnets with host numbers: 60, 60, 60, 30, 30 2 bits in subnetid? No, only 4 subnets. 3 bits in subnetid? No, at most 32 hosts per
subnets.
62 hosts
62 hosts
30 hosts
62 hosts
30 hosts
First mask (26 1s)
255.255.255.192
Second mask (27 1s)
255.255.255.224router
Notes-4 CMPT 471 2003-3 20
Subnet Addressing (cont.)
Subnet broadcasting Subnet broadcast address
hostid is all 1s 3-tuple form: {netid, subnetid, -1}, “-1”
means “all 1s”. {netid, -1, -1}
Means “deliver packet to all hosts with network address netid, even if they are in separate physical subnets”
Operationally, such broadcasting make sense only if routers that interconnect the subnets agree to propagate the packets to all subnets
Notes-4 CMPT 471 2003-3 21
Classless Addressing Also called supernetting Combine several address blocks to create
a larger address range: supernet Instead of using a single netid for
multiple subnets, it allows a network’s addresses to span multiple netids E.g., an organization that needs 1000 addresses
can be granted 4 class C blocks instead of 1 class B block
X.Y.32.0 ~ X.Y.32.255X.Y.33.0 ~ X.Y.33.255X.Y.34.0 ~ X.Y.34.255X.Y.35.0 ~ X.Y.35.255
Notes-4 CMPT 471 2003-3 22
Classless Addressing (cont.)
Address block assigning
Choose address blocks randomly The routers outside of the supernet treat
each block separately Each router has N entries in its routing table,
N = # of blocks; therefore, increase the size of the routing table tremendously
Notes-4 CMPT 471 2003-3 23
Classless Addressing (cont.)
Choose address blocks based on a set of rules so that each router has only one entry in the routing table: required by CIDR (Classless Inter-Domain Routing)
# of blocks is a power of 2 (1, 2, 4, 8 …) The size of each block is a power of 2 The blocks are contiguous in the address
space (no gaps between the blocks) The size of supernet = (# of blocks) *
(size of each block): a power of 2 The first address can be evenly divisible
by supernet size
Notes-4 CMPT 471 2003-3 24
Classless Addressing (cont.)
Blocks defining in different addressing schemesBlock: [first address, last address] Classful address
one block, default mask is always known the first address only can define the block
Subnetting the first address in the subblock (subnet) and
subnet mask define the subblock Supernetting
the first address of the supernet and supernet mask define the superblock
IP address & supernet mask = first address (network address)
Notes-4 CMPT 471 2003-3 25
Classless Addressing (cont.)
Supernet mask The reverse of a subnet mask Has less 1s than the default mask for this class
Subnet mask Divide 1 network into 8 subnets
11111111 11111111 11111111 111 00000
Default mask (class C)
11111111 11111111 11111111 000 00000
Supernet mask Combine 8 networks into 1 supernet
11111111 11111111 11111 000 000 00000
Subnetting
Supernetting
Notes-4 CMPT 471 2003-3 26
Classless Addressing (cont.)
Supernet mask examples A supernet is made out of 16 class C
blocks, what is its supernet mask?
Block #: 16 = 24
Change the last 4 1s in the default mask (class C) to 0s to get the supernet mask:11111111 11111111 11111111 00000000
11111111 11111111 11110000 00000000
Notes-4 CMPT 471 2003-3 27
Classless Addressing (cont.)
A supernet with mask 255.255.248.0 includes an address 205.16.37.44, what is the address range?
First address
205.16.37.44 AND 255.255.248.0 = 205.16.32.0(11001101 00010000 00100000 00000000)
Mask 11111111 11111111 11111000 00000000, 1s: 21, 0s: 11
Last address: 205.16.39.255 (11001101 00010000 00100111 11111111)
Notes-4 CMPT 471 2003-3 28
Classless Addressing (cont.)
Slash notation (CIDR notation): A.B.C.D/n
For identifying a CIDR block A.B.C.D: an IP address n: # of bits that are shared in every
address in the block, i.e., # of 1s in the mask
Prefix: common part of the address range (similar to netid), prefix length = n
Suffix: varying part of the address range (similar to hostid), suffix length = 32 - n
Notes-4 CMPT 471 2003-3 29
Classless Addressing (cont.)
Relationship between mask and prefix length
/n Mask /n Mask
/1 128.0.0.0 /12 255.240.0.0
/2 192.0.0.0 /16 255.255.0.0 (class B)
/3 224.0.0.0 /24 255.255.255.0 (class C)
/8 255.0.0.0 (class A) /32 255.255.255.255
Class A: a.b.c.d/8Class B: a.b.c.d/16Class C: a.b.c.d/24
Notes-4 CMPT 471 2003-3 30
Classless Addressing (cont.)
Subnetting with classless addressing Increase supernet prefix length (n) to
define the subnet prefix length
Example: an organization is granted the block 130.34.12.64/26. It needs to have 4 subnets. What is the subnet address and address range for each subnet?
Prefix length = 26, suffix length = 6 # of addresses in the block: 26 = 64
4 subnets 16 addresses per subnet 4 subnets subnet prefix /28 (2 more 1s in
the mask)
Notes-4 CMPT 471 2003-3 31
Classless Addressing (cont.)
Subnet address ranges 1st: 130.34.12.64/28 ~ 130.34.12.79/28 2nd: 130.34.12.80/28 ~ 130.34.12.95/28 3rd: 130.34.12.96/28 ~ 130.34.12.111/28 4th: 130.34.12.112/28 ~ 130.34.12.127/28
130.34.12.64/28
130.34.12.80/28
130.34.12.96/28 130.
34.1
2.11
2/28
R3
R1
R2
Site: 130.34.12.64/26
Notes-4 CMPT 471 2003-3 32
Classless Addressing (cont.)
Reserved CIDR blocks Private addresses, unroutable addresses Used with private networks Never assigned to networks in the global
Internet Router in the global Internet knows they
are reserved addresses, and can detect it if a packet destined to the reserved address accidentally reaches the Internet
Notes-4 CMPT 471 2003-3 33
Classless Addressing (cont.)
Reserved CIDR blocks: list
Prefix First address Last address
10/8 10.0.0.0 10.255.255.255
172.16/12 172.16.0.0 172.31.255.255
192.168/16 192.168.0.0 192.168.255.255
169.254/16 169.254.0.0 169.254.255.255