Transcript DHCP - Chabot College

Chabot College
ELEC 99.05
IP Address Distribution Methods
CISCO NETWORKING ACADEMY
IP Address Distribution
Methods:
•
•
•
•
Manual Host Configuration
RARP
BOOTP
DHCP
CISCO NETWORKING ACADEMY
Manual Host Configuration
• Required elements:
– IP Address
– Subnet Mask
– Default Gateway
• Typical additional elements:
– Domain Name Server Address
– Default Domain Suffix
CISCO NETWORKING ACADEMY
Windows Host Configuration
• On IP Address tab:
– IP Address
– Subnet Mask
CISCO NETWORKING ACADEMY
Windows Host Configuration
• On Gateway tab:
– Default Gateway
CISCO NETWORKING ACADEMY
Windows Host Configuration
• On DNS
Configuration tab:
– Domain Name
Server Address
– Default Domain
Suffix
– Host Name
(used in dynamic
DNS)
CISCO NETWORKING ACADEMY
RARP
• RARP, or Reverse Address Resolution
Protocol.
• Like ARP, used to map MAC address
to IP addresses.
• Works in reverse - used by devices to
find their own IP address.
• Typically not used on PCs.
CISCO NETWORKING ACADEMY
RARP
• What kind of device would not know its
own IP address?
CISCO NETWORKING ACADEMY
Diskless Workstations
• Diskless workstations have no
permanent storage (like a hard drive)
to store network configurations.
• Diskless workstations will know their
own MAC address because it’s burned
in to the card, but they have to use
RARP to find their IP.
CISCO NETWORKING ACADEMY
RARP request
• Diskless workstations will send a RARP
request, which is a Layer-2 broadcast.
FF-FF-FF-FF-FF-FF
CISCO NETWORKING ACADEMY
RARP reply
• Only a RARP server can respond to a
RARP request.
• RARP servers maintain a table of IP to
MAC address mappings for RARP
clients.
• During the boot process, RARP clients
call the RARP server to obtain their IP
configuration information.
CISCO NETWORKING ACADEMY
BOOTP
• Bootstrap Protocol
• Specified in RFC 951
• Like RARP, maps IP address to MAC
addresses.
• Mappings maintained in a data table
on a BOOTP server.
• Used for PCs
CISCO NETWORKING ACADEMY
BOOTP
• Client sends a bootrequest packet - a
Layer 3 broadcast (255.255.255.255)
• BOOTP server responds with a
bootreply packet containing client’s IP
address and gateway IP address.
CISCO NETWORKING ACADEMY
BOOTP
• Much more convenient than manual
configuration
• Neverthless, requires tedious
maintenance of bootp table:
hamilton
burr
101-gateway
mjh-gateway
welch-tipa
welch-tipb
1
1
1
1
1
1
02.60.8c.06.34.98
02.60.8c.34.11.78
02.60.8c.23.ab.35
02.60.8c.12.32.bc
02.60.8c.22.65.32
02.60.8c.12.15.c8
• Today, DHCP has largely replaced
BOOTP.
CISCO NETWORKING ACADEMY
36.19.0.5
36.44.0.12
36.44.0.32
36.42.0.64
36.47.0.14
36.46.0.12
DHCP
• Dynamic Host Configuration Protocol
• Specified in RFC 2131
• Based on the BOOTP protocol.
Adds automatic allocation of reusable
addresses.
• Widely used for PCs.
• Most ISPs use DHCP for residential
customers.
CISCO NETWORKING ACADEMY
DHCP
• Like BOOTP:
– Client sends an IP broadcast datagram.
– DHCP server returns packet containing IP
address of client and gateway.
CISCO NETWORKING ACADEMY
DHCP
• Unlike BOOTP:
– Server can return additional information
and provide a complete IP configuration:
• Subnet mask
• Domain Name Server address
– Addresses are drawn from a shared pool,
so 15 intermittent users can compete for
10 available addresses - an advantage.
CISCO NETWORKING ACADEMY
DHCP
• Unlike BOOTP:
– Addresses are dynamic - they may
change each time a host boots.
– Addresses are granted for a limited
period of time - a “lease” time.
– When a lease expires, the address is
available to another host.
– By default, hosts attempt to renegotiate
and extend their lease when 50% of the
lease time is up.
CISCO NETWORKING ACADEMY
DHCP Lease
• Run winipcfg on your
curriculum PC
• Click the
More Info>> button
• When does your
current lease expire?
CISCO NETWORKING ACADEMY
Configuring Windows Hosts
for DHCP
• Select
Obtain an “IP address
automatically”
• Most DHCP servers
return a complete
TCP/IP
configuration.
• You’re done!
CISCO NETWORKING ACADEMY
DHCP Server Failure
• Most networks have a centralized
DHCP server. What happens if it fails?
– Hosts that are running will continue until:
• their lease expires
• they shut down
– Hosts that boot up will fail to connect to
the network because they cannot obtain
an IP address.
CISCO NETWORKING ACADEMY
DHCP & Network Structure
• Remember, the DHCP Discover packet
is a Layer 3 broadcast packet, with
address 255.255.255.255.
• By default, routers do not forward
broadcast traffic.
CISCO NETWORKING ACADEMY
DHCP & Network Structure
• How will the client’s DHCP Discover
packet reach the centralized DHCP
server over a routed network?
B ro a d c a s t tra ffic
s to p s he re
N e tw o rk A
D HC P
S e rve r
H o s t1
CISCO NETWORKING ACADEMY
H o s t2
H o s t3
DHCP & Network Structure
• The router can be configured to
forward broadcast traffic only on a
particular port (a Layer 4 concept).
Configured to
foward port 68
broadcasts
Network A
DHCP
Server
Host1
CISCO NETWORKING ACADEMY
Host2
Host3
DHCP - Benefits
• Low maintenance
• Provides complete IP configuration
• Easy to renumber your network - just
change the address range covered by
the DHCP server
CISCO NETWORKING ACADEMY
DHCP - Tradeoffs
• DHCP server presents a “single point
of failure”
• DHCP cannot be a complete solution
because...
CISCO NETWORKING ACADEMY
Address Allocation
• Do we want every address to be
allocated in this dynamically by DHCP?
• Which types of hosts should have
addresses that are static - that never
change?
CISCO NETWORKING ACADEMY
Address Allocation
• What about servers?
• What would happen if a server had a
different address each day you try to
reach it?
• Clearly, some hosts should have static
addresses that don’t depend on a
DHCP server.
CISCO NETWORKING ACADEMY
IP Host Address Standards
DHCP is great for workstations, but not
so great for servers or fixed parts of the
network (e.g. the default gateway)
CISCO NETWORKING ACADEMY
IP Host Address Standards
• Typical standards provide an address
range for:
– Network infrastructure & devices
(e.g. routers, default gateway)
– Hosts with fixed IP addresses
– Hosts with addresses served automatically
by DHCP
– Printers or other special device classes
– Testing
CISCO NETWORKING ACADEMY
CLPCCD IP Host Standards
Host #
0-9
Use
Infrastructure (Network devices)
10-29
IP Hosts w/ static addresses
30-199
DHCP-Served Clients
200-229
Unassigned (use for Lab PCs)
230-239
Printers
240-249
Test Addresses
250-254
Reserved
CISCO NETWORKING ACADEMY
.