lars@ACC-SB-UNIX.ARPA.UUCP (01/31/87)
A private communication mentioned that > The current 32-bit address is divided into 4 fields (N,H,L,I): > > | N | H | L | I | > |--------|----------|--------|--------| > > The proposed division (N,H1,H2,H3,F,G,L1,I1,I2) is according to > that which is to be proposed by BBN and the DDN PMO in an upcoming > RFC expected out in the January timeframe: > > | N |H1 H2|H3 F|G|L1|I1| I2 | > |-------|---------|------------|-------| > > The F field is proposed as a flag to indicate a X.25 Logical Address; > the G field is reserved for future use. With Logical Addressing > the H field is proposed to be expanded to 10 bits (H1,H2,H3) and the > I field similarily (I1,I2). With Logical Addressing, there will > remain a 16-bit X.25 Logical Address (H,I). This means there are > 4 Internet Addresses which map to the same X.25 Logical Address > (those which differ only in the H3,L1, and I1 fields). For > compatibility with existing DDN X.25 the H field must still be > greater than 64 for all X.25 Logical Addresses. This prompts a few questions: (1) Current address formats: I can see in HOSTS.TXT that currently class A networks based on C/30 PSN's use the above format, mostly with N = network number (1-126) H = port number on PSN L = 0 (can be non-zero with a port expander) I = PSN number in network I believe that class B networks based on BBN PSN's use N,H = network number (128.1 - 191.255) L = port number on PSN I = PSN number in network I don't know what format is used for class C PSN's. (2) Logical addressing. As I understand it, the above physical addressing scheme is used throughout ARPAnet and MILNET; other networks use "logical addressing". Software generally has to be configured to know which of the two schemes is in use. When a host comes up, the PSN tells it its address if the network is physical, but the host tells the PSN its address if the network is logical. Physical and logical addressing cannot be mixed in the same network. (3) IP addresses versus AHIP addresses. Logical addressing is a feature of the 1822 interface protocol (AHIP) and there is a standard translation from IP to AHIP. (4) IP addresses versus X.25 addresses The conversion between IP addresses and X.25 addresses is defined in the DDN X.25 spec. so long as the class A "L" field is not used. Use of bits in the IP address to trigger a request for logical addresses is a new feature that may invalidate current port expander implementations. (5) RFC review. BBN and the DDN PMO are discussing this, but no decision will be made until the RFC has had a chance to elicit comments from the protocol research community ? Did I get all of that right ? Lars Poulsen, ACC Customer Service