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CLASSFUL IPv4 ADDRESSES +
DATAGRAM FORWARDING
Internet Protocols
CSC / ECE 573
Fall, 2005 N. C. State University copyright 2005 Douglas S. Reeves (^) 2
Today’s Lecture
I. IPv4 Addresses
II. Address Classes
III. “Special Case” Addresses
IV. Forwarding Basics
V. Forwarding Decisions
VI. Next-Hop vs. Destination Addresses
IPv4 ADDRESSES
copyright 2005 Douglas S. Reeves (^) 4
How Do Addresses Get Assigned?
1. ICANN (Internet Corp. for Assigned Numbers and
Names)
- establishes policy for address and name allocation
- Allocates top-level address space to regional registries
2. Regional registries allocate address space to
ISPs, companies, and other organizations
- APNIC (Asia-Pacific)
- ARIN (North America )
- RIPE (Europe)
- LACNIC (Latin America and Caribbean)
3. Sys admins assign individual host addresses
copyright 2005 Douglas S. Reeves (^) 5
IP Allocation Goals (RFC 2050)
1. Conservation: fair distribution of globally unique
Internet address space, no stockpiling
2. Routability: distribution in a hierarchical manner,
makes routing easier
3. Public registries document address space
allocation and assignment
copyright 2005 Douglas S. Reeves (^) 6
How Do I Get to www.ietf.org?
(…some hops omitted…) 24.93.64. 66.15.132. 66.185.152. 66.185.139. 66.185.145. 152.63.43. 152.63.41. 152.63.39. 152.63.39. 157.130.44.
www.ietf.org 132.151.6. DNS translates this to… User specifies destination of … Router forwarding tables determine the path is…
copyright 2005 Douglas S. Reeves (^) 7
IPv4 Addresses
- 32-bits long, globally unique
- Each interface has an IP address H IP IP Example: a router R IP1 …. IP Example: a multi-homed host network A network B copyright 2005 Douglas S. Reeves (^) 8
Dotted Decimal Notation
Dotted decimal representation 152. 1. 54. 48 32-bit address 10011000 00000001 00110110 00110000 8 bits 8 bits 8 bits 8 bits
- A convenient way to describe (and remember)
IPv4 addresses
- Example IPv4 ADDRESS CLASSES copyright 2005 Douglas S. Reeves (^) 10
Classful Addresses
- Addresses are organized in a two-level hierarchy 1.the network part (leftmost, most significant) 2.the host part (rightmost, least significant) 32- x bits Network ID Host ID
- More networks (= larger network part) means
fewer hosts per network (= smaller host part), and
vice versa
x bits copyright 2005 Douglas S. Reeves (^) 11
Classful Address Formats
1 0 Network ID Host ID 7 24 E
D
C
B
A
Class 2 10 Network ID Host ID 14 16 3 110 Network ID Host ID 2 1 8 4 1110 Multicast Address 2 8 5 11110 reserved 2 7 copyright 2005 Douglas S. Reeves (^) 12
Classful Address Ranges
- The size (number of bits) in the network part is not
fixed
- the first few bits of the address indicate this size
- Classes
- A = addresses 0.0.0.0—127.255.255.
- B = addresses 128.0.0.0—191.255.255.
- C = addresses 192.0.0.0—223.255.225.
- D = addresses 224.0.0.0—239.255.255.
- E = addresses 240.0.0.0—255.255.
copyright 2005 Douglas S. Reeves (^) 19
Directed Broadcast Addresses
- An IP destination address with
Host ID part = all 1’s
means “all hosts attached to the specified network”
- Ex.: Packet sent to address 128.10.255.255 from
host H5 will reach H1…H
H
128.10.2. network 128.10.0. H 128.10.2. H 128.10.2. H 128.10.2. 192.5.48.3 (^) network 192.5.48. H 192.5.48. copyright 2005 Douglas S. Reeves (^) 20
Limited Broadcast Addresses
- An IP destination address
== all 1’s
means “all hosts part of the same network as me”
- Ex.: Packet sent to 255.255.255.255 from host H
reaches H1—H
H
128.10.2. network 128.10.0. H 128.10.2. H 128.10.2. H 128.10.2. 192.5.48. network 192.5.48. H 192.5.48. copyright 2005 Douglas S. Reeves (^) 21
Another Special Case
- An IP source address with
network ID part = all 0’s
means “from this network”
- Only allowed at startup (during bootstrapping)
- allows a machine to communicate temporarily before it learns its own IP address
- thereafter it must not use network 0 copyright 2005 Douglas S. Reeves (^) 22
The Loopback Address
- An IP destination address with
network ID part = all 1’s
means “this computer” (i.e., the one sending the
packet)
- Used in testing network applications without
sending data over a network
- ex.: “ping 127.0.0.1” should always get a reply!
- a datagram with destination address 127.x.x.x should never appear on any network copyright 2005 Douglas S. Reeves (^) 23
Summary of Special Addresses
Destination Destination -- For Address of Type… Broadcast address for same network as originating host All 1’s All 1’s Broadcast address for All 1’s the specified network Anything other than all 0’s or all 1 ’s The address of the All 0’s whole network Anything other than all 0’s or all 1 ’s Then this means … And Host part is … If Network part is… “This computer” Anything (source of the packet) 127 (Class A, all Destination 1 ’s) (host which doesn’t yet know what network it is attached to) Anything other than all 0’s or all 1’s Source All 0’s copyright 2005 Douglas S. Reeves (^) 24
RFC 3330: Special-Use IPv4 Addresses
- 0.0.0.0—0.255.255.255 "This" Network [RFC1700]
- 10.0.0.0—10.255.255.255 Private-Use Networks [RFC1918]
- 24.0.0.0—24.255.255.255 Cable Television Networks
- 169.254.0.0—169.254.255.255 Link Local
- 172.16.0.0—172.23.255.255 Private-Use Networks [RFC1918]
- 192.168.0.0--192.168.255.255 Private-Use Networks [RFC1918]
- 224.0.0.0—239.255.255.255 Multicast [RFC3171]
- 240.0.0.0—255.255.255.255 Reserved for Future Use [RFC1700]
FORWARDING BASICS copyright 2005 Douglas S. Reeves (^) 26
Routers and Neighbors
- Routers (also called Gateways )
- receive packets on one network, send out on another
- Neighbors (or directly-connected computers)
- are attached to the same physical network
- can communicate directly with each other (i.e., no router needed) network 192.5.48.
R
152.14.51. 192.5.48. network 152.14.0. H 192.5.48.
H
192.5.48.
H
152.14.51. R 152.14.51. 192.5.48. copyright 2005 Douglas S. Reeves (^) 27
Packet Forwarding
- Deciding what neighbor to send a packet to is a
forwarding decision
- Ex.: for H1 to send a packet to H2, should it
forward the packet to…
- 192.5.48.12 (router R1)
- or 192.5.48.3 (router R2)? R 152.14.51. 192.5.48. network 152.14.0. network 192.5.48.
H
192.5.48.
H
192.5.48.
H
152.14.51. R 152.14.51. 192.5.48. copyright 2005 Douglas S. Reeves (^) 28
Direct Packet Delivery
- Host H x wishes to send packet to a neighboring
host H y
- how does H x know they are on the same network?
- H x frames (encapsulates) the datagram according
to the requirements of the network connecting H x
and H y
- H x sends this frame directly to H y
- there are no intervening routers involved copyright 2005 Douglas S. Reeves (^) 29
Indirect Datagram Delivery
- Needed if hosts Hx and Hy are not neighbors
- Q: how does Hx figure this out?
- Hx picks a neighboring router R1 to forward the
datagram to
- Hx frames the packet, sends directly to R copyright 2005 Douglas S. Reeves (^) 30
Indirect Datagram Delivery (cont’d)
- R1 extracts the packet, picks a neighboring router
R2 to forward to, frames the packet, sends to R
- ...
- Rn extracts packet, determines Hy is a neighbor
(how does Rn know this?), frames the packet,
sends directly to Hy
copyright 2005 Douglas S. Reeves (^) 37
Other Consequences
- Forwarding (generally) does not consider…
- application type
- quality of service requirements
- bandwidth available
- congestion
- reliability
- …! copyright 2005 Douglas S. Reeves (^) 38
“Default” Routes
- Frequently, a single router R is used for most
outgoing traffic
- may need to specify a few destination-specific network routes
- “everything else” goes through R copyright 2005 Douglas S. Reeves (^) 39
“Default” Routes (cont’d)
- In the forwarding table, there will be an entry with
key = "all other (non-specified) destination
networks“
- normal meaning: “the rest of the Internet”
- simplifies forwarding tables copyright 2005 Douglas S. Reeves (^) 40
“Host-Specific” Routes
- The key may be a single destination host address
- allows specifying a route to a single computer
- Useful for
- testing and debugging purposes
- security purposes
- what else? copyright 2005 Douglas S. Reeves (^) 41
The “Datagram Forwarding” Algorithm
/* M is a machine (router or host) making / / a forwarding decision about a packet */ Extract destination address Hd, compute network part N if (N matches any directly connected networks) deliver to Hd directly else if (there is a host-specific route for Hd) forward datagram to specified next hop else if (there is a route for network N) forward datagram to specified next hop … copyright 2005 Douglas S. Reeves (^) 42
The “Datagram Forwarding” Algorithm
(cont’d)
… else if (there is a default route) forward datagram to default router else /* Hd is not directly connected and we / / don’t know how to get to it… */ discard the datagram and declare routing error
copyright 2005 Douglas S. Reeves (^) 43
Host Forwarding Tables
- Hosts also need forwarding tables to pick the
appropriate "first hop" router
R
40.0.0. 30.0.0. network 40.0.0. network 30.0.0.
H
30.0.0. R 30.0.0. 20.0.0.6 (^) network 20.0.0.
- Frequently there is only one directly-connected
router, and the only entry in the table is the default
route
copyright 2005 Douglas S. Reeves (^) 44
Example
copyright 2005 Douglas S. Reeves (^) 45
The “Datagram Receiving” Algorithm
if (Hd is one of M’s IP addresses) receive the datagram else if (Hd is a limited or directed broadcast address for the network on which it was sent) receive the datagram else if (M is a router) forward the datagram if possible else /* M is a host and this packet is not intended for it */ discard the datagram copyright 2005 Douglas S. Reeves (^) 46
Should Multi-Homed Hosts Forward?
- Since they don’t participate in routing protocols…
probably not!
- inefficient routes
- can create loops
- leads to broadcast "storms“
- etc. DESTINATION vs. NEXT-HOP ADDRESSES copyright 2005 Douglas S. Reeves (^) 48
Destination vs. Next Hop IP Addresses
- The destination IP address in a IP datagram never
changes
- At router R, the datagram is framed and a physical
address is added to get it to the "next hop router"
