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                                                                     EXHIBIT 5.1
October 16, 1996
 
Board of Directors of Penril DataComm Networks, Inc.
c/o Henry D. Epstein
Chairman of the Board
Penril DataComm Networks, Inc.
1300 Quince Orchard Boulevard
Gaithersburg, MD 20878
 
Gentlemen:
 
     You have requested our opinion as to whether (i) the proposed distribution
("Spin-off") by Penril DataComm Networks, Inc. ("Penril") to its shareholders of
100% of the outstanding stock of Access Beyond, Inc., a wholly-owned subsidiary
of Penril, will qualify as a tax-free spin-off pursuant to Section 355 of the
Internal Revenue Code of 1986, as amended (the "Code"), and (ii) the merger (the
"Merger") of Beta Acquisition Corp. with and into Penril, pursuant to which the
outstanding shares of common stock of Penril will be converted into the right to
receive common stock of Bay Networks, Inc. will qualify as a reorganization
within the meaning of Section 368(a)(1)(B) of the Code.
 
     In rendering this opinion, we have (with your permission) relied upon and
assumed as correct and complete the factual representations and information
contained in (i) the Management Certificates (attached hereto), (ii) the Plan
and Agreement of Merger entered into as of June 16, 1996, as amended on August
5, 1996, by and among Beta Acquisition Corp., Bay Networks, Inc. and Penril and
schedules thereto and (iii) a certain letter from Bay Networks, Inc. to Penril,
all dated as of June 16, 1996, and all other information, data, documentation
and materials presented to us during the course of our evaluation of the
Spin-off and Merger. We have also assumed that the Spin-off and the Merger
contemplated by such Plan and Agreement of Merger will be consummated as set
forth therein and as described in the preliminary proxy materials of Penril
dated August 8, 1996 and that the factual representations and material described
above will continue to be correct and complete as of the dates of the Spin-off
and the Merger. We have not undertaken to independently verify any of such facts
or information.
 
     Based on the foregoing, in our opinion it is more likely than not that (i)
the Spin-off will qualify as a nontaxable distribution under Section 355 of the
Code and (ii) the Merger will qualify as a reorganization within the meaning of
Section 368(a)(1)(B) of the Code.
 
     This opinion expresses our views only as to the federal income tax laws in
effect on the date hereof, including the Code, applicable treasury regulations,
published rulings, administrative practices of the Internal Revenue Service
("IRS") and published court decisions. No opinion is expressed with respect to
state, local, foreign or other tax laws. Moreover, this opinion does not relate
to or purport to cover any matters other than the ones expressly stated herein.
 
     This opinion represents our legal judgement as to the matters addressed
herein, but is not binding on the IRS (which may not agree with this opinion) or
any court. Accordingly, no assurance can be given that the opinion expressed
herein will not be contested by the IRS. Similarly, no assurance can be given
that the opinion expressed herein, if contested by the IRS, would be sustained
by a court. Furthermore, the authorities upon which we have relied are subject
to change, whether prospectively or retroactively, and any variation or
difference in the information presented to us might affect the conclusions
stated herein. Also, if the tax-free nature of the Merger is invalidated, it
would reduce the likelihood that the Spin-off will qualify as a tax-free
spin-off pursuant to Section 355 of the Code. We assume no obligation to revise
or supplement this opinion should the present federal income tax laws be changed
by any legislation, judicial decisions, or otherwise.
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     This opinion is rendered only to Penril in connection with the filing of a
Registration Statement on Form S-4 by Bay Networks, Inc. and may not be relied
upon by any other person, firm or entity for any purpose without our express
written consent. We consent to the filing of this letter as an exhibit to the
Registration Statement and to being named in the Proxy Statement/Prospectus
under the heading "Legal Matters" as counsel for Penril. Except for the
discussion of "Certain Federal Income Tax Consequences" in the Proxy
Statement/Prospectus forming a part of the Registration Statement, this letter
may not be paraphrased, quoted or summarized, nor may it be duplicated or
reproduced in part.
 
                                          Very truly yours,
                                          /s/ Benesch, Friedlander, Coplan &
                                          Aronoff P.L.L.
 
                                          BENESCH, FRIEDLANDER,
                                          COPLAN & ARONOFF P.L.L.