# IP Addressing

IP Addressing and Subnetting Review

Class A: 1-126/8
Class B: 128-191/16
Class C: 192-223/24
Class D: 224-239
Class E: 240-255

Static NAT

Does not help with IP address conservation, but makes and inside server available to client on the Internet.

Router(config)# interface ethernet0/0
Router(config-if)# ip nat inside
Router(config)# interface serial0/0
Router(config-if)# ip nat outside
Router(config)# ip nat inside source static 10.1.1.1 200.1.1.1

Dynamic NAT Without PAT

Like static NAT but uses a pool of addresses to assign dynamically.

Dynamic NAT and PAT Configuration

Router(config)# interface ethernet0/0
Router(config-if)# ip nat inside
Router(config)# interface serial0/0
Router(config-if)# ip nat outside
Router(config)# access-list 1 permit 10.1.1.0 0.0.0.255
Router(config)# ip nat pool NATPOOL 200.1.1.1 200.1.1.2 netmask 255.255.255.252
Router(config)# ip nat inside source list 1 pool NATPOOL

Command Reference

Router(config)# ip nat inside destination list {access-list-number | name} pool name
Router# show ip nat statistics
Router# show ip nat translations
Router# clear ip nat translation ?

IP Header

- Version (4b)
- Header Length (4b): Defines the length of the IP header, including optional fields.
- DS Field (8b): Differentiated Services Field, originally called Type of Service (ToS).
- Packet Length (16b): Defines the length of the IP packet, including the data.
- Identification (16b): All fragments of the original packet contain the same identifier.
- Flags (3b): Fragmentation flags.
- Fragment Offset (13b): Identifies the fragment location in the original packet.
- Time to Live (4b)
- Protocol (4b): Identifies the contents of the data field.
- Header Checksum (16b): Frame Check Sequence of the IP header (not the data).
- Source IP Address (32b)
- Destination IP Address (32b)
- Optional Header Fields and Padding (multiple of 4Bytes): For future expansion via optional headers.

IP Protocol Field Values

- ICMP: 1
- TCP: 6
- UDP: 17
- EIGRP: 88
- OSPF: 89
- PIM: 103