No need to operate a DHCP server. If a server s IP address changes, clients will lose the ability to access it!

Transcription

1 setting IP addresses copyright 2015 Robert Montante Static Address Assignments Suitable for small networks No need to operate a DHCP server Necessary for server systems If a server s IP address changes, clients will lose the ability to access it! Accidentally giving two hosts the same address, or specifying an incorrect subnet mask, can isolate a host or even break the network 1

2 Setting static IP addresses Linux the "ifconfig" command can be used: > ifconfig eth Windows the "netsh" command can be used: > netsh interface ip set address name="local Area Connection" static GUI tools are also available Choosing IP Addresses Internet Assigned Numbers Authority (IANA) manages IP address blocks IANA assigns blocks to RIRs RIRs assign sub-blocks to specific organizations for their own use or for redistribution (ISPs do this) ISPs assign sub-blocks, individual IP addresses to customers Some IP address blocks are reserved for special uses e.g /8 always refers back to the local host known as the loopback address 2

3 RIRs Regional Internet Registries RIRs receive blocks of IP addresses from IANA assign sub-blocks to ISPs, other organizations IP Address Map - October

4 IP Address Map - October 2012 Distribution of 420 Million Sampled IP Addresses, June - October

5 Daily Internet Usage Map - October 2012 Special IP addresses from RFC 6890, 2013 Address Block CIDR mask / / / / / / / / / / / / / / / / / /4 Address Range Uses - RFC Refers to "This" network (only used) Private-use networks - non-routable Public-Data Network Cable TV networks Shared address space (for Carrier-Grade NAT) (similar to Private-use address spaces) Loopback addresses Link Local (e.g. APIPA, used in Windows in XP) Private-use networks - non-routable IETF protocol assignments DS-Lite (Dual-Stack, IPv4-and-IPv6 devices) "TEST-NET-1" (e.g. example addresses in documentation) 6to4 relay anycast Private-use networks - non-routable Benchmark tests of network interconnect devices "TEST-NET-2" (e.g. example addresses in documentation) "TEST-NET-3" (e.g. example addresses in documentation) Multicast (former class D) Reserved (former class E, "experimental") IP "universal broadcast" address (not routed or used) 5

6 DHCP Dynamic Host Configuration Protocol Operation: Starting host broadcasts DHCP Discovery packet Local gateway should route this packet to one or more DHCP servers DHCP servers respond with DHCP Offer Offer contains IP address, subnet mask, and some other information Host acknowledges with DHCP Request Server returns acknowledgement of a DHCP lease with a fixed lifetime typically a week or so, or a day or less in public areas Host can renew its lease periodically as long as it wants it Renewals typically occur when the lease is half over example Windows DHCP configuration 6

7 example of a dhcpd.conf file (OpenBSD) DHCP exploration activity Start wireshark (on the physical host) Display filter: eth.addr==<your MAC address> Release your DHCP-assigned IP address: Linux: dhclient -r Windows: ipconfig /release Obtain a new DHCP lease: Linux: dhclient -4 Windows: ipconfig /renew Do you see the DHCP exchange? 7

8 subnetting Dividing an IP Address - Two Approaches The original approach IP classes Five network classes, determined by numeric value class A: the biggest networks; only 126 such class B: medium-size networks; 16,384 total class C: small networks; over 2 million possible class D, class E: special uses Wasteful and limiting Since 1993 Classless Internet Domain Routing (CIDR) IP addresses divided based on network mask 8

9 IP Address Classes (the old way) class A: to class B: to class C: to class D: to class E: to CIDR Classless Internet Domain Routing Network address blocks use as many network bits as needed remaining bits are host bits note: the fewer the network bits, the bigger the network Network masks (a.k.a. subnet masks) mark off network bits Network administrator can subdivide a network into subnets by lengthening the subnet mask 9

10 CIDR Notation Network addresses include the network mask information class A addresses are written / /8 class B addresses are written / /16 class C addresses are written / / / 16 (former class B network) /24, /25, /25 these are three subnets of /16 Subnetting Start with a network block: /16 (only part of the 2 nd private address range) Decide what's needed: Need so-many subnets? Need so-many hosts-per-subnet? Determine number of bits needed to count each subnet or each host-on-a-subnet Move bits from host ID to network ID to according to subnet/hosts needs Define corresponding subnet mask 10

11 Subnetting example Network block / hhhhhhhh hhhhhhhh network mask: = subnets needed so, need enough bits to count at least = 128 >= 75, so 7 bits needed to count subnets Move 7 leftmost host-id bits into network ID nnnnnnnh hhhhhhhh 9 bits left available to count hosts within a subnet New subnet mask: = Subnetting example subnet addresses Network ID bits: 16 (original block) + 7 (subnetting) = = 128 possible subnets Subnetworks: x.0/23 where x varies from one subnet to the next Subnet mask: Host ID bits: = = 510 hosts per subnet 1 st subnet: = /23 2 nd subnet: = /23 3 rd subnet: = /23 4 th subnet: = /23 last subnet: = /23 11

12 Subnetting example 2 Network block / hhhhhhhh hhhhhhhh network mask: = hosts needed per subnet so, need enough host bits to count at least = 1024 >= 1000, so 10 bits needed to count hosts 10 rightmost host-id bits leave 6 left over for subnet IDs nnnnnnhh hhhhhhhh New subnet mask: = Subnetting example 2 subnet addresses Network ID bits: 16 (original block) + 6 (subnetting) = = 64 possible subnets Subnetworks: x.0/22 where x varies from one subnet to the next Subnet mask: st subnet: = /22 2 nd subnet: = /22 3 rd subnet: = /22 4 th subnet: = /22 last subnet: = /22 12

13 a DHCP server with subnetting Network/Subnet Addresses; Host Addresses, Broadcast Addresses IP address divided into network address and host address For a given network/subnet, the network address is fixed Host bits: Host bits are all 0s: the IP address is the address of the entire subnet Host bits are all 1s: the IP address is the subnet's broadcast address Thus, h host bits yield (2 h - 2) valid host addresses 13

14 done 14