IPv4

IPv4 is Internet Protocol version 4, deployed for production in ARPANET in 1983. The latest IETF standard is defined by RFC 791. It is connectionless, best effort delivery protocol over packet-switched networks. It does not guarantee delivery, nor does it prevent the duplicates or out-of-order packets. It protects its header by a checksum but not the payload. The length of the packet may vary and the original packet may be fragmented if necessary into smaller packets, which in their turn might be further fragmented.

IPv4 packet structure:

ECN stands for Explicit Congestion Notification (RFC 3168), must be supported by both ends and only effective if underlying network supports it.

Id is used to identify a group of fragments belonging to the same IP datagram.

Flags are used for the fragmentation as well. Bit 0 is reserved and must to 0. Bit 1, if set, means Don’t Fragment. Bit 2, if set, means More Fragments to follow.

TTL is decremented by each router and the packet is dropped when its TTL = 0 with ICMP Time Exceeded message is usually sent to a sender. This feature is used by traceroute.

IPv4 addresses

Initially the first octet was used as a network identifier and the other 3 octets as host ones. This only allowed 256 networks. Then 5 network classes were defined:

• Class A with the first bit of the first octet set to 0. This allowed 2^7 class A networks, each with 2^24 - 2 hosts.
• Class B with the first bit of the first octet set to 1 and two octets used for networks. Therefore, there could be 2^15 class B networks, each with 2^16 - 2 hosts in each one.
• Class C with the first and the second bits in the first octet set to 1 and three octets used for networks. 2^22 networks and 2^8 - 2 hosts in each one.
• Class D with the first three bits in the first octet set to 1. This space is used for multicast addresses.
• Class E with the first four bits in the first octet set 1. This address space is reserved.

Classes A, B and C are referred as classful networks. The scheme lacked the flexibility and to overcome IPv4 address pool exhaustion, Classless Interdomain Routing (CIDR) was created. It allows subnetworks of any size.

CIDR is managed by Internet Assigned Numbers Authority (IANA) and Regional Internet Registries (RIRs). Each RIR has its own publicly searchable WHOIS database of IP addresses assignments.

Special Addresses

Address Resolution

There are two types of IP addresses mapping: to 12-bit MAC addresses and to DNS names. ARP and RARP protocols are used for the first type and DNS - for the second.