rjwelsh@CCT.BBN.COM ("Robert J. Welsh") (02/04/87)
Lars Poulson writes: >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. PSNs (Packet Switch Nodes - formerly IMP) do not know about IP leader classes. Only gateways and hosts know about IP. Class C IP address: |110|---------------------|--------| 21 bit net no. local host To use logical addr. with this leader limits the addressing range. When a gateway translates the IP addr to 1822 or whatever, it must supply the upper 8 bits of the logical name (a constant) and use "local host" as the lower 8 bits of the name. This leaves you with a address range of 377 (octal) hosts. > >(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. Not true. Physical and Logical addressing can coexist on the same network. Furthermore, we can even support physical and logical addressing hosts from the same IMP. > >(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. Logical Addressing requires the 1822L leader. > >(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 <*>